The Palette/purple/Taken from an animal

Tyrian purple

Squeezed out of the glands of sea snails at roughly ten thousand snails per gram, and restricted by Roman law to the emperor's own cloak.

Also called royal purple · imperial purple · murex purple · argaman · tekhelet (the blue variant)

Hex: #66023C
RGB: 102, 2, 60
First used: c.1600 BC
From: Tyre and Sidon, in present-day Lebanon

What it is

Mainly 6,6′-dibromoindigo: indigo with two bromine atoms attached, which is what shifts it from blue to purple.

How it is made

The dye comes from the hypobranchial gland of several sea snails, chiefly Murex brandaris and Murex trunculus. The gland is removed, or the whole snail is crushed for small species, and the secretion is left in the sun and air where enzymes and light convert a colourless precursor into the dye.

Then it is fermented. Pliny describes a process of steeping the glands in salt for days and simmering the liquor in lead or tin vessels for a week and a half, skimming constantly, reducing it by more than half. The smell was so bad that dyeworks were legally pushed to the edges of cities, and the stink is recorded as clinging permanently to the cloth. A purple garment announced itself before it was seen.

The yield is the story. Estimates run around ten thousand snails for a gram of dye, and the mounds of crushed shells outside ancient Tyre and Sidon are archaeological features you can still identify. Nothing about the price was artificial scarcity. It was just arithmetic.

Tyrian purple, Natural dye extracted from Murex sea snails. Wikipedia's description, not ours.

The history

The Phoenicians built a trade empire on it. The Greek name Phoinikes may itself come from the word for the dye. It was worth several times its weight in gold, and unlike almost every other natural dye it got better with light and washing rather than worse, which made it worth the money to anyone who could find the money.

Rome turned it into law. Sumptuary legislation restricted purple by rank, and under some emperors wearing the full imperial purple was treason punishable by death. The word 'born in the purple' comes from this, and so does the purple of cardinals and of academic gowns, and so, eventually, does purple meaning 'premium' on a packaging brief three thousand years later.

Production collapsed with the fall of Constantinople in 1453 and the knowledge was effectively lost. A handful of people have reconstructed it since. Mohamed Ghassen Nouira in Tunisia works snails by hand and the videos below show the whole process. The colour was also, for a long time, thought to be a lost cause for identifying tekhelet, the blue of Jewish ritual fringes, which comes from the same snail genus processed differently.

Where it is used

Imperial and ecclesiastical dress, the borders of togas by rank, luxury textiles across the Mediterranean. Its afterlife is in every institution that still uses purple to mean authority: cardinals, universities, the House of Lords.

For anybody choosing a brand colour, this is the origin of purple-means-luxury, and it is worth knowing how thoroughly the meaning has been eaten. Purple signalled money because it was ruinously expensive and legally restricted. Now it is a free hex value, mauve was the first industrial dye in 1856, and purple on a shelf reads as budget confectionery as often as it reads as premium. A meaning built on scarcity does not survive the scarcity ending.

Go and look at it

Shell middens at Tyre and Sidon
Lebanon
Mounds of crushed murex shells, still identifiable. The industrial waste of a colour.
Byzantine imperial portraits
San Vitale, Ravenna, in situ mosaics
Justinian and Theodora in purple, with everybody else in white and gold. The hierarchy is the palette.

How it behaves

Made of: Mainly 6,6′-dibromoindigo: indigo with two bromine atoms attached, which is what shifts it from blue to purple.
Opacity: Transparent. It stains rather than covers, so it can be glazed over another colour.
Lightfastness: Permanent. It will outlive the thing it is painted on.

Unusually, it improves with exposure and washing rather than fading.
The shade ranges from deep red-purple to near-black depending on species and process.
The same snails yield blue rather than purple if the dyeing is done in sunlight, which is the basis of tekhelet reconstructions.

The numbers

All derived from the hex, never typed beside it

HEX: #66023C; What a browser and a design file want.
RGB: 102, 2, 60; The same three numbers in base ten. Light, not ink.
HSL: 325°, 96%, 20%; Hue, saturation, lightness, how a person describes a colour.
CMYK: 0, 98, 41, 60; Arithmetic only. A real separation depends on the press, the paper and a profile.
LAB: 21, 44, -6; Perceptual. L* of 21 is how light this actually looks, which HSL gets wrong.

This hex is an approximation, and a page about pigment has to say so. Tyrian purple 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 325°

Its opposite, by the diagram

#66023C

#02662C

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

#66023C

#7D2859

#944E77

#B3819E

#D1B3C5

#E8D9E2

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

#66023C

#570233

#47012A

#33011E

#1F0112

#0F0009

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
12.81:1 · AAA

Handgloves

Black on it
1.64:1 · Fails

Handgloves

It on paper
12.16:1 · AAA

Handgloves

It on ink
1.43:1 · 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.