Another thought: if languages didn’t differentiate between blue and green until late, maybe it could be, after all, that we didn’t really perceive the difference until later in our evolution - or that people in certain areas didn’t.
-JT
Could be but that would also mean that various cultures went through the genetic changes related to that perception at different times, whereas the linguistic differentiation could have just been cultural - similar to the eskimos having 20 or 30 words for ice.