The Tinted Cube Illusion
What to see and do: The small center squares on the front faces of the toy are identical in the two pictures. One looks yellow and the other looks blue but they are in fact both gray. If you don't believe it, drag both squares to the white area to see their true colors.
What's happening: The left picture was tinted blue and the right picture was tinted yellow. Your brain adjusts the colors to try to figure out what the colors would look like without the blue and yellow tints.
Credit: This illusion is based on similar illusions done by Dale Purves and R. Beau Lotto of Duke University. See http://www.purveslab.net/seeforyourself/.