Tired eyes
Troxler fading
Stare at the cross for 20 seconds without moving your eyes at all. What happens to the colors?
They fade away, until the whole picture looks plain gray. Move your eyes and they come straight back.
Why? Your eyes are always making tiny movements so that cells see changes. Soft, blurry colors don’t change enough when they move, so the cells stop reporting them and your brain fills in plain gray.
Picture: Bautsch, CC0 · Wikipedia(opens in a new tab)