This image has no color, right? Mouseover the picture and stare
intently at the colored boxes. There's no need to focus on any particular
spot, just look at the boxes for a few minutes. Move your mouse away
and look at the first image again. The horizontal gratings will look
greenish and the vertical gratings pink. That's not especially
impressive, but come back tomorrow and the effect will still occur.
It's not a simple afterimage. Print out the black and white grid and carry
it around with you. Rotate it and see what happens.
No one's sure what's behind this sensory phenomenon; some think it temporarily modifies the cells in
the visual cortex that respond to color and orientation.