Two Greys
A grey square on black and a grey square on white.
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A grey square on black and a grey square on white.
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Light is judged against its neighbours
Both squares are the same grey. How light a surface looks depends less on the light it sends to your eye than on how it compares with what surrounds it: against black the grey stands out as light, against white as dark.
This is simultaneous contrast, one of the oldest effects in the study of colour; the chemist Michel Eugène Chevreul wrote the classic account in 1839, after dye customers complained that colours woven next to each other changed.