Two squares, A & B, are marked in the illustration. How much darker is the one compared to the other?
You may (or may not) be surprised to hear that they don't differ – the two squares are the same shade of gray (their RGB values are 120:120:120, give or take a bit since the image is slightly dithered by the JPG compression).
So what's going on? At least two effects are at work. The first is the visual cue of a chess board, suggesting that A and B are black and white squares, thus setting up the expectation that they are supposed to differ. The second effect is more beautiful: square B seems to lie in the shadow of the green pillar, suggesting that it is darkend, which suggests that in reality, it should actually be a bit brighter than it appears. Your brain obliges, and dutifully modifies the equally-bright (120:120:120) incoming photons, rendering them lighter.
The exploded diagram below should convince you. At step 1, A & B should appear different, but by the third and fourth steps, their similarity should be obvious.
nothing more to see. please move along.