Indigo
 
A piece of indigo plant dye from India,
about 6 cm (2.5 in) square
Spectral coordinates
Wavelength420–450[1](disputed) nm
Frequency~714–670 THz
About these coordinates     Color coordinates
Hex triplet#330099
sRGBB (r, g, b)(51, 0, 153)
HSV (h, s, v)(260°, 100%, 60%)
CIELChuv (L, C, h)(20, 73, 270°)
SourceHTML/CSS[2]
ISCC–NBS descriptorDark violet
B: Normalized to [0–255] (byte)

Indigo is a term used for a number of hues in the region of blue. The word comes from the ancient dye of the same name. The term "indigo" can refer to the color of the dye, various colors of fabric dyed with indigo dye, a spectral color, one of the seven colors of the rainbow, or a region on the color wheel, and can include various shades of blue, ultramarine, and green-blue. Since the web era, the term has also been used for various purple and violet hues identified as "indigo", based on use of the term "indigo" in HTML web page specifications.

The word "indigo" comes from the Latin word indicum, meaning "Indian", as the plant-based dye was originally exported to Europe from India.

The Early Modern English word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic color-naming system.[3]

The first known recorded use of indigo as a color name in English was in 1289.[4]

Isaac Newton regarded indigo as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between blue and violet; however, sources differ as to its actual position in the electromagnetic spectrum. Later scientists have concluded that what Newton called "blue" was what is now called cyan or blue-green; and what Newton called "indigo" was what is now called blue.

In the 1980s, programmers produced a somewhat arbitary list of color names for the X Window computer operating system, resulting in the HTML and CSS specifications issued in the 1990s using the term "indigo" for a dark purple hue. This has resulted in violet and purple hues also being associated with the term "indigo" since that time.

History

Indigo as a dye

Extract of natural indigo applied to paper

Idigo dye is a blue color, obtained from several different types of plants. The indigo plant (Indigofera tinctoria) often called "true indigo" probably produces the best results, although several others are close in color: Japanese indigo (Polygonum tinctoria), Natal indigo (Indigofera arrecta), Guatemalan indigo (Indigofera suffruticosa), Chinese indigo (Persicaria tinctoria), and woad Isatis tinctoria.

Indigofera tinctoria and related species were cultivated in East Asia, Egypt, India, Bangladesh and Peru in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BC and comes from Huaca Prieta, in contemporary Peru.[5] Pliny the Elder mentions India as the source of the dye after which it was named.[6] It was imported from there in small quantities via the Silk Road.[7]

The Ancient Greek term for the dye was Ἰνδικὸν φάρμακον (indikon pharmakon, "Indian dye"), which, adopted to Latin as indicum (a second declension noun) or indico (oblique case) and via Portuguese, gave rise to the modern word indigo.[8]

Indigo extracted from woad

In early Europe the main source was from the woad plant Isatis tinctoria, also known as pastel.[9] For a long time, woad was the main source of blue dye in Europe. Woad was replaced by "true indigo", as trade routes opened up. Plant sources have now been largely replaced by synthetic dyes.

Spanish explorers discovered an American species of indigo and began to cultivate the product in Guatemala. The English and French subsequently began to encourage indigo cultivation in their colonies in the West Indies.[10]

In North America, indigo was introduced by Eliza Lucas into colonial South Carolina, where it became the colony's second-most important cash crop (after rice).[11] Before the Revolutionary War, indigo accounted for more than one-third of the value of exports from the American colonies.[12]

Isaac Newton's classification of indigo as a spectral color

Indigo is one of the colors on Newton's color wheel.

Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England,[13] supplanting the homegrown woad as source of blue dye. In a pivotal experiment in the history of optics, the young Newton shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this optical spectrum, Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."[14] He linked the seven prismatic colors to the seven notes of a western major scale,[15] as shown in his color wheel, with orange and indigo as the semitones. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:

Newton's observation of prismatic colors: Comparing this to a color image of the visible light spectrum shows that "blue" corresponds to cyan, while "indigo" corresponds to blue.

I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks.[16]

Traditional seven colors of the rainbow

Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonics "Richard of York gave battle in vain" and Roy G. Biv. James Clerk Maxwell and Hermann von Helmholtz accepted indigo as an appropriate name for the color flanking violet in the spectrum.[17]

Later scientists concluded that Newton named the colors differently from current usage.[18][19] According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name blue-green or cyan."[20] If this is true, Newton's seven spectral colors would have been:

Red:  Orange:  Yellow:  Green:  Blue:  Indigo:  Violet: 

The human eye does not readily differentiate hues in the wavelengths between what are now called blue and violet. If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to Isaac Asimov, "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes, it seems merely deep blue."[21]

1800s

In 1821, Abraham Werner published Werner's Nomenclature of Colours, where indigo, called indigo blue, is classified as a blue hue, and not listed among the violet hues. He writes that the color is composed of "Berlin blue, a little black, and a small portion of apple green," and indicating it is the color of blue copper ore, with Berlin blue being described as the color of a blue jay's wing, a hepatica flower, or a blue sapphire.[22]

According to an article, Definition of the Color Indigo published in Nature magazine in the late 1800s, Newton's use of the term "indigo" referred to a spectral color between blue and violet. However, the article states that Wilhelm von Bezold, in his treatise on color, disagreed with Newton's use of the term, on the basis that the pigment indigo was a darker hue than the spectral color; and furthermore, Professor Ogden Rood points out that indigo pigment corresponds to the cyan-blue region of the spectrum, lying between blue and green, although darker in hue. Rood considers that artificial ultramarine pigment is closer to the point of the spectrum described as "indigo", and proposed renaming that spectral point as "ultramarine". The article goes on to state that comparison of the pigments, both dry and wet, with Maxwell's discs and with the spectrum, that indigo is almost identical to Prussian blue, stating that it "certainly does not lie on the violet side of 'blue.'" When scraped, a lump of indigo pigment appears more violet, and if powdered or dissolved, becomes greenish.[23]

Modern spectral classification

Several modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers,[1][24][25] which lies on the short-wave side of color wheel (RGB) blue, towards (spectral) violet.

The correspondence of this definition with colors of actual indigo dyes, though, is disputed. Optical scientists Hardy and Perrin list indigo as between 445[26] and 464 nm wavelength,[27] which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards azure.

Other modern color scientists, such as Bohren and Clothiaux (2006), and J.W.G. Hunt (1980), divide the spectrum between violet and blue at about 450 nm, with no hue specifically named indigo.[28][29]

Web era

Origin of "Indigo" as a name for purple in web pages

Towards the end of the 20th century, purple colors also became referred to as "indigo". In the 1980s, computer programmers Jim Gettys, Paul Ravelling, John C. Thomas and Jim Fulton produced a list of colors for the X Window Operating System. The color identified as "indigo" was not the color indigo (as generally understood at the time), but was actually a dark purple hue; the programmers assigned it the hex code #4B0082  . This collection of color names was somewhat arbitrary: Thomas used a box of 72 Crayola crayons as a standard, whereas Ravelling used color swabs from the now-defunct Sinclair Paints company, resulting in the color list for version X11 of the operating system containing fanciful color names such as "papaya whip", "blanched almond" and "peach puff". The database was also criticised for its many inconsistencies, such as "dark gray" being lighter than "gray", and for the color distribution being uneven, tending towards reds and greens at the expense of blues.[30]

In the 1990s, this list which came with version X11 became the basis of the html and CSS color rendition used in websites and web design. This resulted in the name "Indigo" being associated with purple and violet hues in web page design and graphic design. Physics author John Spacey writes on the website Simplicable that the X11 programmers did not have any background in color theory, and that as these names are used by web designers and graphic designers, the name indigo has since that time been strongly associated with purple or violet. Spacey writes, "As such, a few programmers accidentally repurposed a color name that was known to civilisations for thousands of years."[31]

Crayola crayon colors

The Crayola company released an indigo crayon in 1999, with the Crayola website using the hex code #4F49C6   to approximate the crayon color. The 2001 iron indigo crayon is portrayed using hex code #184FA1  , the 2004 indigo crayon color uses #5D76CB  , the 2019 iridescent indigo uses #3C32CD  .

Distinction among tones of indigo

Like many other colors (orange, rose, and violet are the best-known), indigo gets its name from an object in the natural world—the plant named indigo once used for dyeing cloth (see also Indigo dye).

The color pigment indigo is equivalent to the web color indigo and approximates the color indigo that is usually reproduced in pigments and colored pencils.

The color of indigo dye is a different color from either spectrum indigo or pigment indigo. This is the actual color of the dye. A vat full of this dye is a darker color, approximating the web color midnight blue.

The color "electric indigo" is a bright and saturated color between the traditional indigo and violet. This is the brightest color indigo that can be approximated on a computer screen; it is a color located between the (primary) blue and the color violet of the RGB color wheel.

The web color blue violet or deep indigo is a tone of indigo brighter than pigment indigo, but not as bright as electric indigo.

Below are displayed several colors now referred to as indigo, some of which have been named as indigo since the adoption of html color names in the world wide web era.

Indigo dye color

Indigo Dye
 
About these coordinates     Color coordinates
Hex triplet#00416A
sRGBB (r, g, b)(0, 65, 106)
HSV (h, s, v)(203°, 100%, 42%)
CIELChuv (L, C, h)(26, 40, 246°)
Source
ISCC–NBS descriptorDeep blue
B: Normalized to [0–255] (byte)

Indigo dye is a greenish dark blue color, obtained from either the leaves of the tropical Indigo plant (Indigofera), or from woad (Isatis tinctoria), or the Chinese indigo (Persicaria tinctoria). Many societies make use of the Indigofera plant for producing different shades of blue. Cloth that is repeatedly boiled in an indigo dye bath-solution (boiled and left to dry, boiled and left to dry, etc.), the blue pigment becomes darker on the cloth. After dyeing, the cloth is hung in the open air to dry.

A Native American woman described the process used by the Cherokee Indians when extracting the dye:

We raised our indigo which we cut in the morning while the dew was still on it; then we put it in a tub and soaked it overnight, and the next day we foamed it up by beating it with a gourd. We let it stand overnight again, and the next day rubbed tallow on our hands to kill the foam. Afterwards, we poured the water off, and the sediment left in the bottom we would pour into a pitcher or crock to let it get dry, and then we would put it into a poke made of cloth (i.e. sack made of coarse cloth) and then when we wanted any of it to dye [there]with, we would take the dry indigo.[32][33]

In Sa Pa, Vietnam, the tropical Indigo (Indigo tinctoria) leaves are harvested and, while still fresh, placed inside a tub of room-temperature to lukewarm water where they are left to sit for 3 to 4 days and allowed to ferment, until the water turns green. Afterwards, crushed limestone (pickling lime) is added to the water, at which time the water with the leaves are vigorously agitated for 15 to 20 minutes, until the water turns blue. The blue pigment settles as sediment at the bottom of the tub. The sediment is scooped out and stored. When dyeing cloth, the pigment is then boiled in a vat of water; the cloth (usually made from yarns of hemp) is inserted into the vat for absorbing the dye. After hanging out to dry, the boiling process is repeated as often as needed to produce a darker color.

Electric indigo

Electric Indigo
 
About these coordinates     Color coordinates
Hex triplet#6F00FF[34]
sRGBB (r, g, b)(111, 0, 255)
HSV (h, s, v)(266°, 100%, 100%)
CIELChuv (L, C, h)(39, 134, 273°)
Source[Unsourced]
ISCC–NBS descriptorVivid purplish blue
B: Normalized to [0–255] (byte)

"Electric indigo" is brighter than the pigment indigo reproduced above. When plotted on the CIE chromaticity diagram, this color is at 435 nanometers, in the middle of the portion of the spectrum traditionally considered indigo, i.e., between 450 and 420 nanometers. This color is only an approximation of spectral indigo, since actual spectral colors are outside the gamut of the sRGB color system.

Deep indigo (web color blue-violet)

Blue-Violet
 
About these coordinates     Color coordinates
Hex triplet#8A2BE2
sRGBB (r, g, b)(138, 43, 226)
HSV (h, s, v)(271°, 81%, 89%)
CIELChuv (L, C, h)(42, 115, 279°)
SourceX11
ISCC–NBS descriptorVivid violet
B: Normalized to [0–255] (byte)

At right is displayed the web color "blue-violet", a color intermediate in brightness between electric indigo and pigment indigo. It is also known as "deep indigo".

Web color indigo

Web color Indigo
 
About these coordinates     Color coordinates
Hex triplet#4B0082
sRGBB (r, g, b)(75, 0, 130)
HSV (h, s, v)(275°, 100%, 51%)
CIELChuv (L, C, h)(20, 62, 279°)
Source
ISCC–NBS descriptorDark violet
B: Normalized to [0–255] (byte)
Web safe Indigo
 
About these coordinates     Color coordinates
Hex triplet#330099
sRGBB (r, g, b)(51, 0, 153)
HSV (h, s, v)(260°, 100%, 60%)
CIELChuv (L, C, h)(20, 73, 270°)
Source
B: Normalized to [0–255] (byte)

The color box on the right displays the web color indigo, the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that is possible to reproduce on a computer screen. Its hue is closer to violet than to indigo dye for which the color is named. Pigment indigo can be obtained by mixing 55% pigment cyan with about 45% pigment magenta.

Compare the subtractive colors to the additive colors in the two primary color charts in the article on primary colors to see the distinction between electric colors as reproducible from light on a computer screen (additive colors) and the pigment colors reproducible with pigments (subtractive colors); the additive colors are significantly brighter because they are produced from light instead of pigment.

Web color indigo represents the way the color indigo was always reproduced in pigments, paints, or colored pencils in the 1950s. By the 1970s, because of the advent of psychedelic art, artists became accustomed to brighter pigments. Pigments called "bright indigo" or "bright blue-violet" (the pigment equivalent of the electric indigo reproduced in the section above) became available in artists' pigments and colored pencils.

Tropical indigo

Tropical Indigo
 
About these coordinates     Color coordinates
Hex triplet#9683EC
sRGBB (r, g, b)(150, 131, 236)
HSV (h, s, v)(251°, 44%, 93%)
CIELChuv (L, C, h)(61, 84, 271°)
SourceGallego and Sanz[35]
ISCC–NBS descriptorVivid violet
B: Normalized to [0–255] (byte)

'Tropical Indigo' is the color that is called añil in the Guía de coloraciones (Guide to colorations) by Rosa Gallego and Juan Carlos Sanz, a color dictionary published in 2005 that is widely popular in the Hispanophone realm.

Imperial blue

Imperial Blue
 
About these coordinates     Color coordinates
Hex triplet#002395
sRGBB (r, g, b)(0, 35, 149)
HSV (h, s, v)(226°, 100%, 58%)
CIELChuv (L, C, h)(21, 72, 263°)
Source[Unsourced]
ISCC–NBS descriptorVivid blue
B: Normalized to [0–255] (byte)

In nature

Birds

Indigo bunting
Indigo bunting

Fungi

An upturned Lactarius indigo mushroom
An upturned Lactarius indigo mushroom

Snakes

Eastern indigo snake
Eastern indigo snake
  • The eastern indigo snake, Drymarchon couperi, of the southeastern United States, is a dark blue/black.

In culture

Business

  • IndiGo is an Indian budget airline that uses an indigo logo and operates only Airbus A320s.
  • Indigo Books and Music uses an indigo logo and has sometimes referred to the color as "blue" in advertising.[36][37]
  • The GameCube was initially released in 2 color variants, including one bearing the title of 'Indigo', with the main console and controllers in that color.

Computer graphics

  • Electric indigo is sometimes used as a glow color for computer graphics lighting, possibly because it seems to change color from indigo to lavender when blended with white.

Dyes

Indigo is created in potholes carved in pumice "tufgrond" in Karoland, Sumatra
  • Indigo dye was used to dye denim, giving the original 'blue jeans' their distinctive colour.
  • The original Postal Worker uniform contained indigo dye, partly due to the dye not running when wet. [38]
  • Guatemala, as of 1778, was considered one of the world's foremost providers of indigo.[39]
  • In Mexico, indigo is known as añil.[40] After silver, and cochineal to produce red, añil was the most important product exported by historical Mexico.[41]
  • The use of añil is survived in the Philippines, particularly in the Visayas and Mindanao. The powder dye is mixed with vinegar to be applied to the cheek of a person suffering from mumps.

Food

  • Scientists discovered in 2008 that when a banana becomes ripe, it glows bright indigo under a black light. Some insects, as well as birds, see into the ultraviolet, because they are tetrachromats and can use this information to tell when a banana is ready to eat. The glow is the result of a chemical created as the green chlorophyll in the peel breaks down.[42]

Literature

Marina Warner's novel Indigo (1992) is a retelling of Shakespeare's The Tempest and features the production of indigo dye by Sycorax.

Military

The French Army adopted dark blue indigo at the time of the French Revolution, as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, Napoleon decided to restore the white coats because of shortages of indigo dye imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914.

In the Better Call Saul episode "Hero", Howard Hamlin mentions that his law firm Hamlin Hamlin & McGill trademarked a colour called "Hamlindigo" whilst confronting Jimmy McGill over trademark infringement in a billboard advertisement he produced for his own legal services.

Spirituality

The spiritualist applications use electric indigo, because the color is positioned between blue and violet on the spectrum.[43]

See also

References

  1. 1 2 Rosen, Joe (26 June 2017). Encyclopedia of Physics. Infobase Publishing. ISBN 9781438110134 – via Google Books.
  2. ↑ W3C TR CSS3 Color Module, SVG color keywords. W3C. (May 2003). Retrieved on 14 December 2007.
  3. ↑ Ottenheimer, Harriet Joseph (2009). The anthropology of language: an introduction to linguistic anthropology (2nd ed.). Belmont, CA: Wadsworth. p. 29. ISBN 978-0-495-50884-7.
  4. ↑ Maerz and Paul A Dictionary of Color New York:1930 McGraw-Hill Page 197; Color Sample of Indigo: Page 117 Plate 47 Color Sample E10
  5. ↑ Splitstoser, Jeffrey C.; Dillehay, Tom D.; Wouters, Jan; Claro, Ana (September 2016). "Early pre-Hispanic use of indigo blue in Peru". Science Advances. 2 (9): e1501623. Bibcode:2016SciA....2E1623S. doi:10.1126/sciadv.1501623. PMC 5023320. PMID 27652337.
  6. ↑ "Night of the Indigo". harappa.com. Retrieved 20 May 2016.
  7. ↑ Clark, Robin J. H.; Cooksey, Christopher J.; Daniels, Marcus A. M.; Withnall, Robert (1993). "Indigo, woad, and Tyrian Purple: important vat dyes from antiquity to the present". Endeavour. 17 (4): 191–199. doi:10.1016/0160-9327(93)90062-8.
  8. ↑ Ἰνδικός in Henry George Liddell. Robert Scott. A Greek-English Lexicon. revised and augmented throughout by. Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford. Clarendon Press. 1940; English indigo since the 17th century, changed from 16th-century indico.
  9. ↑ "Getting the blues: the pastel trade in southwest France". Life on La Lune. 22 May 2011. Retrieved 23 February 2018.
  10. ↑ Pritchard, James (2004). In Search of Empire: The French in the Americas, 1670–1730. Cambridge: Cambridge University Press. p. 127.
  11. ↑ Eliza Layne Martin. "Eliza Lucas Pinckney:Indigo in the Atlantic World" (PDF). Archived from the original (PDF) on 7 June 2010. Retrieved 24 August 2013.
  12. ↑ "Eliza Lucas Pinckney" Archived 21 November 2008 at the Wayback Machine, Biographies, National Women's History Museum, 2007, accessed 7 December 2008.
  13. ↑ Allen, O.N. Allen & Ethel K. (1981). The Leguminosae: a source book of characteristics, uses, and nodulation (null ed.). Madison, Wisc.: University of Wisconsin Press. p. 343. ISBN 978-0-299-08400-4.
  14. ↑ Newton's draft of A Theory Concerning Light and Colors on newtonproject.sussex.ac.uk
  15. ↑ "SHiPS Resource Center || Newton's Colors". Archived from the original on 29 September 2014. Retrieved 16 October 2010.
  16. ↑ Brewster, David (1855). Memoirs of the life, writings and discoveries of Sir Isaac Newton, Volume 1. p. 408.
  17. ↑ Ronchi, Lucia R.; Jodi Sandford (2009). The Excentric Blue. An Abridged Historical Review. Fondazione Giorgio Ronchi. ISBN 978-88-88649-19-1.
  18. ↑ Evans, Ralph M. (1974). The perception of color (null ed.). New York: Wiley-Interscience. ISBN 978-0-471-24785-2.
  19. ↑ McLaren, K. (March 2007). "Newton's indigo". Color Research & Application. 10 (4): 225–∠229. doi:10.1002/col.5080100411.
  20. ↑ Waldman, Gary (1983). Introduction to light : the physics of light, vision, and color (2002 Dover revised ed.). Mineola: Dover Publications. p. 193. ISBN 978-0-486-42118-6.
  21. ↑ Asimov, Isaac (1975). Eyes on the universe : a history of the telescope. Boston: Houghton Mifflin. p. 59. ISBN 978-0-395-20716-1.
  22. ↑ Werner, Abraham (1821). Werner's Nomenclature of Colours. London. p. 41. Retrieved 30 October 2023.
  23. ↑ "Definition of the Color Indigo". Littel's Living Age. 145 (1869). 1880. Retrieved 30 October 2023. Newton denoted by the name of "indigo" the tint of the spectrum lying between "blue" and "violet." Von Bezold, in his work on color, rejects the term, justifying his objection by observing that the pigment indigo is a much darker hue than the spectrum tint. Prof. O. N. Rood, who follows Von Bezold in rejecting the term, brings forward the further objection that the tint of the pigment indigo more nearly corresponds in hue (though it is darker) with the cyan-blue region lying between green and blue. By comparing the tints of indigo pigment, both dry and wet, with the spectrum, and by means of Maxwell's disks, it appears that the hue of indigo is almost identical with that of Prussian blue, and certainly does not lie on the violet side of "blue." Indigo in the dry lump, if scraped, has, however, a more violet tint; but if fractured or powdered, or dissolved, its tint is distinctly greenish. Prof. Rood considers that artificial ultramarine corresponds much more nearly to the true tint of the spectrum at the point usually termed "indigo," and he therefore proposes to substitute the term "ultramarine" in its place, the color of the artificial pigment being thereby intended.
  24. ↑ Group, The HURIS. "Spectrum of Electromagnetic Radiation ( EMR )". www.huris.com. Archived from the original on 28 January 2020. Retrieved 14 June 2013.
  25. ↑ "VIBGYOR Color Segmentation – File Exchange – MATLAB Central". www.mathworks.com. June 2023.
  26. ↑ "What Wavelength Goes with a Color?". Archived from the original on 24 November 2016. Retrieved 24 November 2016.
  27. ↑ Arthur C. Hardy and Fred H. Perrin. The Principles of Optics. McGraw-Hill Book Co., Inc., New York. 1932.
  28. ↑ Craig F. Bohren and Eugene E. Clothiaux (2006). Fundamentals of Atmospheric Radiation. Wiley-VCH. ISBN 978-3-527-40503-9.
  29. ↑ J. W. G. Hunt (1980). Measuring Color. Ellis Horwood Ltd. ISBN 978-0-7458-0125-4.
  30. ↑ Tveten, Julianne (11 October 2015). ""Tomato" versus "#FF6347"—the tragicomic history of CSS color names". Ars Technica. Retrieved 11 October 2015.
  31. ↑ Spacey, John (19 June 2020). "19 Types of Indigo". Simplicable. Archived from the original on 5 June 2023. Retrieved 30 October 2023. In 1986 some programmers created a list of color names for a unix system known as X11. Having no background in color theory, they placed indigo as a dark purple. This list was later used by HTML and CSS standards that remain in place to this day. These standards are used by millions of designers and digital artists such that the color name indigo is now strongly associated with dark purple or violet. As such, a few programmers accidentally repurposed a color name that was known to civilizations for thousands of years. ...Note the difference between Web Indigo and Indigo. This standard color name is completely detached from the traditional color. This misrepresentation resulted from the random selection by a programmer working on an operating system in 1986.
  32. ↑ Knight, Oliver (1956–57), "History of the Cherokees, 1830–1846", Chronicles of Oklahoma, Oklahoma City: Oklahoma Historical Society, p. 164, OCLC 647927893
  33. ↑ Foreman, Grant (1934). The Five Civilized Tribes. Norman: University of Oklahoma Press. p. 283. ISBN 978-0-8061-0923-7.
  34. ↑ "What Color is Indigo?". 16 December 2014. Archived from the original on 26 December 2015.
  35. ↑ Gallego, Rosa; Sanz, Juan Carlos (2005). Guía de coloraciones (Gallego, Rosa; Sanz, Juan Carlos (2005). Guide to Colorations) Madrid: H. Blume. ISBN 84-89840-31-8
  36. ↑ "It's New and It's Blue" (Indigo advertisement), The Globe and Mail, Toronto, 1 October 1999, p. A3
  37. ↑ "Indigo Bookstore had a 'Think Blue' campaign back in 1999" according to: "Think Blue 2008: a Before and After Tale of Silly Turf Battles and Redemptive Communication". Archived from the original on 10 May 2012. Retrieved 4 February 2013.
  38. ↑ Hejna, Jamie (19 February 2023). "What Color is Indigo?". Freelance Faucet.
  39. ↑ Kitchin, Thomas (1778). The Present State of the West-Indies: Containing an Accurate Description of What Parts Are Possessed by the Several Powers in Europe. London: R. Baldwin. p. 30.
  40. ↑ Gallego, Rosa; Sanz, Juan Carlos (2001). Diccionario Akal del color. Akal. ISBN 978-84-460-1083-8.
  41. ↑ Article „añil“ in: Enciclopedia de México, vol 1, Mexiko-City: Secretaría de Educacion Pública, 1987
  42. ↑ Zurer, Rachel. "Three Smart Things About Banana Peels". Wired. Retrieved February 28, 2020.
  43. ↑ Tansley, David W. Subtle Body: Essence and Shadow 1984 (Art and Cosmos Series--Jill Purce, editor)
  44. ↑ Stevens, Samantha. The Seven Rays: a Universal Guide to the Archangels. City: Insomniac Press, 2004. ISBN 1-894663-49-7 pg. 24
  45. ↑ Graham, Lanier F. (editor) The Rainbow Book Berkeley, California:1976 Shambala Publishing and The Fine Arts Museums of San Francisco (Handbook for the Summer 1976 exhibition The Rainbow Art Show which took place primarily at the De Young Museum, but also at other museums) Indigo Pages 152–153 The color indigo is stated to represent intuition.
  46. ↑ Bailey, Alice A. (1995). The Seven Rays of Life. New York: Lucis Publishing Company. ISBN 978-0-85330-142-4.
  47. ↑ Oslie, Pamalie Life Colors: What the Colors in Your Aura Reveal Novato, California:2000--New World Library Indigo Auras: Pages 161–174
  48. ↑ "Magical Properties of Colors". Wicca Living. Retrieved 28 January 2021.
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