You probably perceived Square B to be lighter, for three reasons:
1) It is surrounded by darker squares.
2) It matches the pattern of lighter squares on the checkerboard.
3) It appears to fall inside the shadow of the blue cylinder.
Wherever there is lighting, a shadow can be cast, which can interfere with color perception.*
Can you see how lighting can have a big impact on the colors we see?
Can you see how that can impact accurate medication selection in your facility?
* Foster DH. Color constancy. Vis Res. 2011;51:647-700.
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