Would you believe me if I told you that the middle brown cube and the middle yellow cube are really the same color?
They really are the same!
Your eyes are playing a trick on you.
If you don't believe me. . . examine it photoshop for yourself.
These pictures are from the Mighty Optical Illusions web-site.
More about this illusion as well as others can be found at http://www.moillusions.com/2008/01/color-tile-illusion-new-aspect.html