Lightness and color constancy refer to the fact that the lightness and colors of objects in the world appear to be relatively stable, even though there may be quite radical changes in the amount of light falling on the scene and the spectrum of the illuminant.

Here are some illusions that ‘catch’ your visual system compensating for what it thinks is a change in illumination (because of a shadow) and illuminant spectrum (because of an overall yellowish or bluish light source):

http://faculty.washington.edu/chudler/afterflag.html

http://www.youtube.com/watch?v=qA2brNUo7WA