Simultaneous contrast
The way a colour seems to change depending on the colours placed next to it.
Also: colour contrast effect
The same grey square looks lighter on black than on white, and slightly greenish on red. This is simultaneous contrast: the eye exaggerates differences between neighbours, pushing each towards the opposite of the other. The French chemist Michel-Eugène Chevreul described it in 1839 after weavers at the Gobelins tapestry works complained that their dyes looked wrong; the dyes were fine, and the neighbours were to blame. The effect is why a colour should be chosen in the setting where it will be used, not on a white card.