Boing Boing presents a couple of nice optical color illusions that you just should see:
Note that square A and square B have the same color.
Here’s a similar illusion with colored squares. The “blue” tiles on the top face of the left cube are the same color as the “yellow” tiles in the top of the right cube.
Don’t take my word for it. Use an image editing program with a eyedropper to see for yourself. I used Photoshop’s eyedropper tool to take 5×5 samples and found that both the “yellow” and the “blue” tiles are C:50 M:40 Y:40 K:5.
The color tile illusion is one of many excellent illusions created by R. Beau Lotto.