Indigo


Indigo is a deep and rich color close to the color wheel blue, as well as to some variants of ultramarine, based on the ancient dye of the same name. The word indigo comes from the Latin for "Indian", as the dye was originally imported to Europe from India.
It is traditionally regarded as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between violet and blue; however, sources differ as to its actual position in the electromagnetic spectrum.
The first known recorded use of indigo as a color name in English was in 1289.

History

Indigofera tinctoria and related species were cultivated in East Asia, Egypt, India, 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 :wikt:indigo|indigo.
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.
Blue dye can be made from two different types of plants: the indigo plant, which produces the best results, and 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, and both plant sources have now been largely replaced by synthetic dyes.

Classification as a spectral color

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. 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.
However, the correspondence of this definition with colors of actual indigo dyes 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.
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, 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:
, while blue corresponds to cyan.
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 conclude 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, cyan or light blue." 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 we today call 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."
Modern color scientists typically divide the spectrum between violet and blue at about 450 nm, with no indigo.

Distinction among the four major tones of indigo

Like many other colors, indigo gets its name from an object in the natural world—the plant named indigo once used for dyeing cloth.
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 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.
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.
Below are displayed these four major tones of indigo.

Electric indigo

"Electric indigo" is brighter than the pigment indigo reproduced below. 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)

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

The color box at right displays the web color indigo, the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above 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 and the pigment colors reproducible with pigments ; 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" became available in artists' pigments and colored pencils.

Tropical indigo

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

Indigo dye

Indigo dye is a greenish dark blue color, obtained from either the leaves of the tropical Indigo plant, or from woad, or the Chinese indigo. 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, 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 and then when we wanted any of it to dye with, we would take the dry indigo.

In Sa Pa, Vietnam, the tropical Indigo 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 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 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.

Imperial blue

In nature

;Birds
;Fungi
;Snakes

In culture

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

Business

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.

Spirituality

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