optical illusion in color perception

I found a site with 3 very incredible optical illusions. No matter what you do, how you look, or where you look, you think you see different colors. BUT they are all the SAME color. The last one is the best, you think you see one yellow and one purple centerpiece, but the 'yellow' and the 'purple' are EXACTLY the same color. Very weird, very cool.

Optical color illusion