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Checkershadow Illusion
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In the figure above, tile A appears to be much darker than tile B, but they are actually of exactly the same shade of gray (click the reveal button). Since a light object in shadow may look the same color as a dark object in the light, our brain bases our perception of an object's brightness (or luminance) on the shadows that are seen.

ReferenceReferences

[1] E. H. Adelson, "Lightness Perception and Lightness illusions", in The New Cognitive Neurosciences, 2nd ed., M. Gazzaniga, Ed. Cambridge, MA: MIT Press, 2000, pp. 339–351.
[2] E. H. Adelson, "Checkershadow Illusion." [Online]. Available: http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html. [Accessed: Mar 9, 2010].