Purple's an interesting color. Because, technically, it doesn't exist. Well, it depends on who you ask, but a physicist will tell you there's no such thing as purple. It's known as a 'non-spectral color', meaning it's not part of the natural spectrum. In a color wheel, the rainbow is looped into a circle, and the ends (red and blue) are linked with purple. But in real life, there's no link. The extreme end of the blue wavelengths is known as violet, but it's really more of a very deep blue.
Linguistically, the term purple is also odd, because for a long time, it didn't exist. Many cultures (both ancient and modern) use the term blue to cover the entire gamut of shades between blue and purple, leading to a whole lot of argument as to exactly what ancient texts are referring to. (More on that later.) But the English term for purple is quite easy to trace; it's first known use was in the Lindisfarne Gospels (see left), written around 700 CE. The term comes from the Latin term for purple murex dye (purpura), which comes from the even older Greek term for the shellfish the dye was made from. The name of the Phonecians, the sea-going people who got rich selling murex dye, translates roughly from Greek to mean "Purple People". Murex was a big freakin' deal in the ancient world, and still is in many scholarly circles.
Before getting into the history of murex - which should probably be a post all its own - a quick word on purple paints in pre-industrialized society. Basically, there aren't any. Very high-quality ultramarine had a purple cast to it, and was sometimes used. Purple shades were also sometimes produced by overpainting blue with a transparent red glaze. And then there was realgar.
Realgar is a crystalline form of arsenic sulfide, and as such, pretty darn toxic. It's worse than orpiment, it's yellow cousin, and orpiment is pretty bad. Realgar is also very rare and hard to find, and isn't quite purple as we know it; it's more a red-violet. When it was available, people did use it for paint. But it was not a common material. This stone was known as 'dragon's blood' among some Mediterranean cultures, and there has been speculation that it was realgar, used as a dye, that is the basis for those legends where a tunic was dipped in dragon's blood and the person who wore it died. (Remember, back them people wore the same clothes day in and day out, often to bed at night also, so we're talking major exposure, not like today's world where we'd wear it once every couple weeks.)
I suspect this lack of purple pigments is why murex was such a big deal.
After doing some research, the surprising thing is, shellfish that produce some kind of snot that turns pinkish purple on exposure to light are not rare. I've found mention of different types scattered all over the world; not just the Med, but in Japan, Central America, England (yes, you read that right - they're in the Bristol Channel), and very likely along the coasts of India. (Or were, before ancient people used them all.) It's not murex, the snail snot, that's the big deal. It was the PROCESS of making Tyrian Purple dye from the murex, that was what the ancient world went ga-ga over. The process was kept secret, of course, and so what with the various wars, sackings, raids, and famines, the secret was eventually lost.
The dye pigment, the actual molecule, is almost identical to the colorant in indigo, and the dye process is eerily similar. Murex shellfish (there were several species in the Med that produced several variants on purple dye - the most expensive cloth was dyed twice, with two different species) were gathered up and put in a vat. Then they were smushed. While it is possible to extract the snail snot WITHOUT killing them (and it is still done that way in Mexico), for the deep purples (and probably blues) of Tyrian Purple, the little critters had to die. After smushing, they were left to rot; the bacteria breaking down the goo removed all the oxygen, just like the urine does in a traditional indigo vat. Then the fabric was dipped, and the fabric would come out green, and gradually change to purple. Just like indigo. The color could be adjusted by exposing the vat to sunlight, and also exposing the wet fabric to sunlight as it dried.
Obviously, this process reeks. One article I read, the author mentions that she went to a seminar about Tyrian Purple dye, and she could smell it the instant she walked in the building - from three floors away. Fabric curators also claim they can tell what was dyed this way very easily; even after a thousand years, the fabric still smells. The dye wasn't limited to fabric, either. The Byzantines used it to color vellum, which was then overwritten with real gold. Talk about conspicuous consumption.
Then there's one of the great dye mysteries of history: Tekhelet, the sacred dye of the Hebrew people. It was once used to color the fringes of prayer shawls, before the secret of the dye was lost; many Jews wear white fringes on their shawls to this day, in remembrance. Modern scholars, unable to resist a puzzle, have been working for years to figure out the lost secret. The first problem was the color itself: The Hebrews used one word to cover both blue and purple. The Talmud warns of counterfiet tekhelet, made from indigo, so it's very likely they really mean blue. The Talmud says the color is from a snail, and we land squarely back at the door of murex. But for many years, no one was able to produce blue dye from the purple/pink snail snot. Then in the 1980s, a scholar realized that with exposure to sunlight, the purple turns to a very pretty blue (that's it up there, being made). An organization was founded to produce the dye and continue the research, and in the computerized world of the 21st century, people have gone back to making a dye almost three thousand years old.
I find that very comforting.