Every one knows that purple was an extremely expensive dye to produce up in ancient times. When did people finally come up with a different recipe for it, and how come no one put together blue and red before then?
Purpura was not "purple" in the modern sense. I think you should find this enlightening and helpful. In point of fact, as explained in the link, some cheaper imitations did mix red and blue dyes, but purpura was not a single shade nor could it really be replicated by the combination of red and blue dyes.