Afterimage
A ghost image in the opposite colour, seen after staring at a strong colour and looking away.
Also: successive contrast
Stare at a red shape for half a minute, then look at a white wall. A pale blue-green shape appears and slowly fades. The cells that respond to red have become tired, so for a moment the others dominate. The afterimage is always roughly the complementary colour of the original. Surgeons' gowns are green or blue partly for this reason: after looking at red tissue, a glance at white would show distracting green ghosts, which a green surface hides.