This post contains a cool optical illusion and a detailed explanation. The centres of the two circles are actually changing colour in synchrony, but your perception is quite different.
This simple relationship between the colors used to create the illusion and the perception of the illusion has helped Shapiro come up with a model of how we perceive it. We see the illusion because our visual system relies not just on color information, but also on contrast information to make judgments about what we are seeing. When color values are plugged into Shapiro’s mathematical model, when the difference between the color angle of the surround and center equals 90 degrees, then the plot of contrast for the left (red) and the right (blue) disk becomes identical, so the centers are seen changing together, while for all other values we see them changing out of phase.