Brilliant red wasn't accessible until the Colombian exchange. It comes predominantly from cochineal beetles, even to this day. (Red 40, anyone?). Blue existed in some forms, but the accessible inexpensive ones were not very bright either. They were typically plant-derived. Mixing what they had access to would create a muddy, brown- or gray-hued purple instead of the brilliant hue of true royal purple.

Brilliant red wasn't accessible until the Colombian exchange. It comes predominantly from cochineal beetles, even to this day. (Red 40, anyone?). Blue existed, but mixing what they had access to would create a muddy, brown- or gray-hued purple instead of the brilliant hue of true royal purple.