The Palette/blue/Grown
Indigo
The blue that is green in the vat, yellow in the bucket, and only becomes blue in the air, and the one Britain ran an empire on.
Also called añil · neel · woad (the European plant)
- Hex
- #00416A
- RGB
- 0, 65, 106
- Pigment index
- NB1 / PB66
- First used
- At least 6,000 years. The oldest dyed textile found so far is Peruvian cotton
- From
- India, and independently Peru, West Africa, Japan, China, Europe
What it is
Indigotin, C₁₆H₁₀N₂O₂, produced from indican in the leaves of Indigofera tinctoria and several unrelated plants.
How it is made
This is the strangest process in colour, because indigo is not soluble and therefore cannot dye anything as it is. The leaves are fermented in water, which releases a soluble precursor; the liquid is then beaten with air, and insoluble blue indigo settles out as a sludge that is dried into cakes. Those cakes are the trade good.
To dye with it you have to undo that. The cake is reduced in an alkaline vat (historically with fermenting fruit, urine or bran, now often with sodium hydrosulfite) until it turns into soluble yellow-green leuco-indigo. Cloth goes into the vat and comes out yellow-green. Then oxygen from the air oxidises it back to insoluble blue, on the fibre, where it is now trapped. The cloth turns blue in your hands as you lift it out. Every dyer describes this moment and they are all correct about it.
Because the dye sits on the fibre rather than in it, indigo rubs and fades. That is not a defect. It is the entire aesthetic of denim: every worn knee and white seam on a pair of jeans is indigo's poor fastness, industrialised into a look.
Indigo, Chemical compound, food additive and dye. Wikipedia's description, not ours.
The history
India was the centre of it. The Greek name indikon means 'from India' and the English word follows. Europe had its own, weaker source in woad, and the two fought a genuine trade war: woad growers in France and Germany got indigo banned as a 'devil's dye' to protect their crop, which worked for about as long as protectionism ever does.
The colonial history is not a footnote. Indigo cultivation under the British in Bengal and Bihar was coercive, planters advanced loans that trapped cultivators into growing dye instead of food on their best land. That produced the Indigo Revolt of 1859–60 in Bengal and, decades later, Gandhi's first major campaign in India at Champaran in 1917. When you look at a pair of jeans you are looking at the end of a supply chain that has an uprising in it.
Adolf von Baeyer worked out the structure in the 1880s and BASF brought synthetic indigo to market in 1897. Within twenty years the Indian natural indigo industry was effectively dead. Almost all indigo now is synthetic and chemically identical; the difference people claim to see in natural indigo comes from the impurities alongside it.
Where it is used
Denim, first and last, the most-worn dyed fabric in human history. Also Japanese aizome and the whole shibori tradition, West African adire and Tuareg tagelmust, Indian block printing and bandhani, and European working clothes for centuries before any of that became fashion.
For a brand, indigo is the rare colour that carries 'ordinary' and 'considered' at once, because it has been the colour of working clothes and of luxury denim without either meaning cancelling the other. It also fades in a way that customers have been trained to read as value rather than wear, which is a trick almost no other colour gets to use.
Go and look at it
Any pair of blue jeans
Everywhere
The cheapest available demonstration of ring-dyeing: cut a worn hem and the fibre is blue outside and white in the middle.
Japanese aizome and shibori textiles
Aizumi Historical Museum, Tokushima; textile collections worldwide
Repeated dips rather than one strong bath. The depth is built in layers, and you can count them.
Champaran, Bihar
India
Where Gandhi's 1917 campaign against forced indigo cultivation began. The colour has a politics.
How it behaves
- Made of
- Indigotin, C₁₆H₁₀N₂O₂, produced from indican in the leaves of Indigofera tinctoria and several unrelated plants.
- Opacity
- Transparent. It stains rather than covers, so it can be glazed over another colour.
- Lightfastness
- Moderate. It will move, noticeably, within decades, faster in thin films and tints.
- Rubs off and fades. This is why denim wears the way it does.
- Dyes without a mordant, which almost no other natural dye manages.
- As a paint pigment it is weak and was largely replaced by Prussian blue in the 18th century.
The numbers
All derived from the hex, never typed beside it
- HEX
- #00416A
- What a browser and a design file want.
- RGB
- 0, 65, 106
- The same three numbers in base ten. Light, not ink.
- HSL
- 203°, 100%, 21%
- Hue, saturation, lightness, how a person describes a colour.
- CMYK
- 100, 39, 0, 58
- Arithmetic only. A real separation depends on the press, the paper and a profile.
- LAB
- 26, -1, -29
- Perceptual. L* of 26 is how light this actually looks, which HSL gets wrong.
This hex is an approximation, and a page about pigment has to say so. Indigo is a substance. A ground mineral scatters light from thousands of separate grains at slightly different angles, and no single sRGB value is that event. It is a flat average of a photograph of it, and the real thing is outside what a screen can emit. Use the number to specify a screen colour. Do not use it to judge the pigment.
Where it sits on the wheel
Hue 203°
Its opposite, by the diagram
The arithmetic opposite, 180° round in HSL. Useful, and not the same as the opposite a painter mixing pigment would name, that wheel was built for paint.
Lighter and darker
Towards white
Mixing with white does not only lighten, it dulls and cools. This is why a pale tint of a brand colour so often looks chalky, and why a designed light shade beats a generated one.
Towards black
Mixing with black kills the hue as it darkens. A better dark is usually the colour plus its neighbour round the wheel, which is what every painter does instead.
Can you set type on it
Handgloves
White on it
10.69:1 · AAA
Handgloves
Black on it
1.96:1 · Fails
Handgloves
It on paper
10.15:1 · AAA
Handgloves
It on ink
1.71:1 · Fails
4.5:1 is the floor for body text and 3:1 for large text, by WCAG. Treat it as a minimum you may not go under rather than as a target: plenty of pairs pass at 4.6:1 and are still unpleasant to read for a page, and the checker will not tell you that.
Watch
Titles and channels as YouTube reports them