Stare at colored stripes and the illusion can last for months

Look at red-and-black stripes then green-and-black stripes for a few minutes, and afterward black-and-white stripes will show faint tinges of the opposite colors — but only at that specific orientation. Discovered by psychologist Celeste McCollough in 1965, the effect has been measured lingering for weeks or even months, far longer than any ordinary afterimage.

— Celeste McCollough, Color Adaptation of Edge-Detectors in the Human Visual System — Science, 1965