This graphic may illustrate a point:
The gray areas are actually all the same color, its in the context of their surroundings that determine perceived differences.
The website I stole this from explains it thus:
"Note, that although the gray rectangles are all of equal luminance, the ones seen in the context with the dark stripes appear brighter than the ones seen in the context with the bright stripes. Note that this effect is opposite to what would be expected from a simple physiological explanation on the basis of simultaneous contrast (in that case the rectangles sharing the long borders with the dark stripes should appear brighter)."
http://www.news-world.us/pics/2009/0...cal-illusions/