Adelson's Checkershadow Illusion
It might be difficult to convince oneself, by looking at the picture, that squares A and B on the chessboard have the same shade of gray. And yet they are exactly the same color! Drag the locators to see that the RGB encoding is the same on both boxes. If you are wary of the computer and the process, you can also check the identity of the colors by printing the image, cutting out the squares A and B and comparing them (or by covering the intermediate squares with your fingers).
 E. H. Adelson, "Lightness Perception and Lightness Illusions," The New Cognitive Neurosciences (M. Gazzaniga, ed.), Cambridge, MA: MIT Press, 2000 pp. 339–351.