The way a colour's appearance shifts because of the colours around it: it looks lighter beside dark, darker beside light, and pushed away in hue from its neighbour.
A perceptual effect of the visual system comparing neighbouring regions rather than judging each in isolation. A grey on a red ground looks faintly greenish; a colour beside its complement looks more saturated.
Nothing about the colour itself changes; a measuring instrument reads it the same in either setting. It is not a display or printing fault.
The same mid-grey square looks lighter on black paper than on white.
It explains why a colour chosen in isolation can look wrong once it sits next to others, in rooms, outfits, charts and make-up alike.