The spinning disk below is a little seizure inducing but a lot of fun because not everyone sees the same color when it spins. Some people see green, others see yellow, a few see red while some see no color at all.
Benham's disk is a simple illusion with a spinning circle, half black, half white streaked with short black lines. There is no color involved, and yet, when the disk spins, people can see bright streaks of it swirling in the top. Why?