The circular area in each quadrant in the left figure is exactly the same color.
The different appearance is due to the surround and the lateral interactions
that occur. On the right I merely changed the background colors so that they
are all identical. You can now see that the circular areas are, indeed, identical.
This phenomenon is called simultaneous color contrast.
Source : The Joy of Visual Perception : http://www.yorku.ca/eye/simcont1.htm