I have seen this illusion many times and for some reason never focused on it. I must have past by this optical illusion a dozen times before finally focusing on an article and reading what it is about. It is very similar to the checkershadow illusion but involves color rather than just shades of gray.
Image Copyright Dale Purves, R. Beau Lotto and Surajit Nundy.
Another interesting tidbit is that the red tiles on top of the left cube are orange and the red tiles on the top of the right cube are purple.
To read more about this illusion read either of these articles: American Scientist: Why We See What We Do and Discover Magazine: Sensory Reflexes.