It’s not that purple dye in general is hard to make (though that’s true), it’s because of one *particular* purple dye.
Tyrian purple, made from a few species of snails off the coast of the eastern Mediterranean, was a deep purple/red dye that did not fade. Its production was held in a monopoly by the Phoenicians, and it required a very large amount of labor to produce, which made it expensive.
Its use was always restricted mostly to priests and kings, but under the Romans and especially the Eastern Romans (Byzantines) it became associated exclusively with the Imperial throne – for example, there was a special room in the Great Palace of Constantinople made of purple stone that empresses would give birth in. This remained even after the creation of Tyrian purple was lost and they started using other dyes.
Almost all associations between royalty and purple trace directly to this association with the Roman emperors.
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