The McCollough effect is when a colorless grating appears as colored contingent on the orientation of the gratings. In layman's terms, If you look at some black lines with a red or purple background, the same pattern on a white background will appear as purple or green.
To induce the effect, all you have to do is stare at the green lines, then the purple, back and forth, for about 30 seconds each.
Vertical lines will only affect other vertical lines, and ditto with horizontal ones, giving a hodgepodge of color on some boxes with lines in them.
The effect occurs even if done with a single eye, meaning that there is evidence of binocular interactions. It's possible that the effect isn't affecting the eye in any way, but instead affecting the brain directly.
If you do end up trying this out on yourself (seriously, don't), a possible way to undo the effects is to merely perform the same actions with an opposite image. Flip the orientation of the lines, reverse the colors, then stare at it for fifteen minutes to put your brain back to basic. But seriously, don't try it.
