This is not an afterimage. Afterimages are brief and they follow your gaze. This one is long, and it stays where it is put. Stare at two coloured gratings for a few minutes, and afterwards black-and-white stripes will carry a colour that is not on the screen. The effect was documented in 1965. It wears off.
This page will change the way you see black-and-white stripes. With the pairing used here, horizontal stripes can come out greenish and vertical stripes pinkish.
The change is temporary. Tested repeatedly, the effect can fade within days; left untested, it has been measured at up to 2.8 months. The effect needs time to become visible: the classic minimum is three minutes, and a shorter session can give a weak result or none at all.
The page keeps no record of your visit. The only thing it changes is your vision.
The grating alternates every six seconds. Keep your eyes near the dot. Small movements are fine. The recommended minimum is three minutes. Longer induction makes a longer effect. You can stop at any time.
Both gratings below are black and white. The screen is emitting no colour. Look from one to the other, then answer.
What do you see?
Your visual system contains cells that respond to edges of a particular orientation together with a particular colour. After a few minutes of the same pairing those cells adjust their response, and black-and-white edges of that orientation carry a tint that is not there. The tint is complementary to the induction colour. This page paired red with horizontal, so horizontal stripes can run greenish, and green with vertical, so vertical stripes can run pinkish.
Celeste McCollough, an American psychologist, described the effect in Science in 1965. Her first demonstrations paired orange and blue; the effect works with any complementary colours, and red and green are the usual classroom version. She reported the effect lasting an hour or more. In 1975 Jones and Holding found that subjects who tested the effect repeatedly lost it within five days, while subjects who left it untested still showed it after 85 days.
Tilt your head 45 degrees and the colour disappears. Straighten up and it returns. The effect is attached to the retina, not to the room, and to the region of the retina that did the looking. In the 1980s, people who spent their days at green-on-black word processors sometimes saw pink text in books afterwards, because the letters had the same spacing as the screen's phosphors.
To be rid of it, check it less. Every test makes it fade faster. The reliable cure is time.
This page cannot confirm what you saw. It has no eyes. The verification is yours.
Sources: Wikipedia, “McCollough effect” (checked 4 August 2026); C. McCollough, “Color Adaptation of Edge-Detectors in the Human Visual System”, Science 149(3688):1115–1116, 1965; P. D. Jones and D. H. Holding, “Extremely long-term persistence of the McCollough effect”, Journal of Experimental Psychology: Human Perception and Performance 1(4):323–327, 1975.