Induction

Viewing two colors at the same time influences both of their appearances. The following is an example of induction, a variation of simultaneous contrast. 

Instructions: In the top half of the diagram below are two identical dark gray patches on a like background. The bottom half of the diagram shows the same dark gray patches on different backgrounds. You will see that they appear different because of their surroundings.

The same phenomenon occurs when colors are used. Note that identical yellow patches in the middle of different colors appear to take on the characteristics of their surroundings.

Other Illusions:
Blind Spots
Blink Effect
Bordering
Chromatic Adaptation
Chromatic Adaptation
Color Blindness
Crispening
Dither Dots
Dithering
Fill Patterns
Opponent After-images
Optical Art
Shape Contrast
Simultaneous Contrast
Simultaneous Contrast
Value Contrast

Other fun stuff:
Free 3D Screensavers
Color Playground
Dimensions of Color
3D Color Puzzle