"When blue dots became rare, participants began to see purple dots as blue," the researchers wrote in the study. Indeed, during the final 200 trials, dots that the participants previously identified as purple now looked blue to them. The participants continued mistaking purple dots for blue ones even when they were specifically warned that the number of blue dots was going to decrease or when they were offered a $10 reward for responding to repeated colors the same way at the end of the study as they did at the beginning of the study.

So, why the sudden change of perception? According to the researchers, it could be that the human brain doesn't make decisions based on cold, hard rules, but rather on prior stimuli. As the balance of blue-to-purple dots shifted, the participants expanded their definition of what "blue" really looked like in order to match the expectations formed from the earlier trials.

https://www.livescience.com/62962-blue-or-purple-dots-illusion.html