Indigo

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 as described by Isaac Newton, 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 naturally based dye was originally exported to Europe from India.
The Early Modern English word indigo referred to the dye, not to the color itself, and indigo is not traditionally part of the basic color-naming system.
The first known recorded use of indigo as a color name in English was in 1289. Due to the extensive knowledge of indigo cultivation by enslaved West Africans, indigo became a major cash crop in the American colonies.
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. Some 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. However, other scholars reject this.
In the 1980s, programmers produced a somewhat arbitrary 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.
Because of the Abney effect, pinpointing indigo to a specific hue value in the HSV color wheel is elusive, as a higher HSV saturation value shifts the hue towards blue. However, on the new CIECAM16 standard, the hues values around 290° may be thought of as indigo, depending on the observer.

History

Indigo as a dye

Indigo dye is a blue color, obtained from several different types of plants. The indigo plant often called "true indigo" probably produces the best results, although several others are close in color: Japanese indigo, Natal indigo, Guatemalan indigo, Chinese indigo, and woad.
Indigofera tinctoria and related species were cultivated in West Africa, 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. Pliny the Elder mentions India as the source of the dye after which it was named. It was imported from there in small quantities via the Silk Road.
The Ancient Greek term for the dye was Ἰνδικὸν φάρμακον, which, adopted to Latin as indicum or indico and via Portuguese, gave rise to the modern word indigo.
In Ancient West Africa,
In early Europe, the main source was from the woad plant Isatis tinctoria, also known as pastel. 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.
In North America, indigo is native to the coastal regions of the American South and was cultivated as early as 1622. Eliza Lucas, whose father sent her indigo seeds as a gift, is credited with introducing the plant to the southern colonies. However, it was enslaved West Africans from present-day Mali, Guinea, Ivory Coast, Senegal, and Nigeria who actually cultivated those indigo seeds and introduced indigo to North America in the form of dye, where it became the colony's second-most important cash crop. Before the Revolutionary War, indigo accounted for more than one-third of the value of exports from the American colonies.

Isaac Newton's classification of indigo as a spectral color

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, 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." He linked the seven prismatic colors to the seven notes of a western major scale, 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:
File:Newton prismatic colours.JPG|thumb|class=skin-invert-image|upright=1.59|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.

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.
Later scientists concluded that Newton named the colors differently from current usage.
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." If this is true, Newton's seven spectral colors would have been:
However, other scholars reject this.
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."

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.
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.

Modern spectral classification

Several modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers, which lies on the short-wave side of color wheel blue, towards 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 and 464 nm wavelength, which occupies a spectrum segment from roughly the color wheel blue extending to the long-wave side, towards azure.
Other modern color scientists, such as Bohren and Clothiaux, and J.W.G. Hunt, divide the spectrum between violet and blue at about 450 nm, with no hue specifically named indigo.

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, 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.
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."