This post isn't talking about the pointillism mentioned yesterday; it's a good primer on the fascinating visual engines of pointillism:
The orange/pink rings in the center of each large ring appear to all be different colors. But actually they are the same. Click on the image to see an animated figure that removes the other colors. What's particularly remarkable about this illusion is that in rings a and c, the color immediately next to the central ring is the same. The only difference between the figures is the frequency at which the rings change color. The same effect occurs between rings b and d.
The central rings in c and d appear to be much closer to each other in color than the central rings in a and b. Monnier and Shevell had several volunteers look at figures like this, comparing the multi-colored rings to single rings on white background. They adjusted the figures on white until they appeared to match the rings in the figure. The responses were consistent: everyone with normal color vision appears to experience the illusion.