Two Greys

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.

• Contrast effect — Wikipedia