A great deal of prestige is attached to the colour purple - the hue of bishops and emperors alike - and, if you've read a little about the ancient world, you'll probably have heard the story explaining why this is the case. The Greeks and Romans - the story goes - knew of only one way of obtaining purple dye: a painstaking and eye-wateringly expensive process involving the extraction of the precious substance from sea snails.
As a boy, I always imagined that the root of this purple business was an inability on the part of the Greeks and the Romans to produce blue dye. I know little about dyes, but the colour blue is notable for occurring in the natural world only very rarely. So it makes sense, at least at first glance, that the ancients would have struggled to produce dye of this colour. The colour purple was the closest that they could come to achieving to such a rarity, hence its prestige. In any case, if they could produce blue dye in quantity, they could just have mixed it with red dye - which has always been one of the cheaper pigments - and every Roman and his dog could have strolled across the forum in lurid purple vestments.
This theory took a bit of a knock a couple of years ago, when I read that the colour purple only became available to the non-fabuously wealthy in 1860 with the invention of magenta pigment (named for the Battle of Magenta of the previous year). Given that, for centuries before that date, the armies of Britain and France marched into battle wearing vivid red and blue uniforms respectively, it seems unlikely that lack of blue pigment was the main contributor to the rarity of purple.
Given that both red and blue dyes were plentiful pre-1860, what, then, was stopping the ancients (or anyone else for that matter) from mixing the two to create purple? Do clothing dyes not mix the way that acrylic paints do?