Weird. Never thought it to be real. In Warhammer 40k, only a populace of a specific planet has those purple eyes. Kinda neat to know that this might have been a super-specific random event (first human settlers of that planet having purple eyes).
I imagine it's a recessive trait, so even if all the first settlers had it, most people a few generations down would not have it, only if there was an external force driving evolution, meaning people with other eye colours couldn't reproduce as well.