Opponent After-images
The nature of our visual system allows us to sometimes see "after-images" which appear once the original stimuli are removed. In the following demonstration, you will see that the colors in after-images are usually the opposite (complementary) colors of the original.
Instructions: Stare at the black spot in the center of the four colored squares for about 30 seconds. Then scroll down and move your gaze to the black spot in the uniform white area. Note the colors of the afterimages relative to the colors of the original stimuli. Did they appear different?

A COUPLE OF GOOD SPOTS FOR OPTICAL ILLUSIONS

MICHAELBACH

COLORCUBE

http://www.electricoyster.com/electric3d/index.html

A COOL FLYING SIM SCREENSAVER SORT OF THING
MOVE CURSOR TO STEER