Why does a color leave a ghost in the opposite color when you look away?
Stare at red long enough and a green ghost follows your eyes onto the next wall.
Your eye senses color with detectors tuned to red, green, and blue. Stare at one strong color and the detectors carrying it get tired, like a tap that has been running, so they send a weaker signal. But your brain does not read each color alone - it reads color by balancing opposites: red against cyan, green against magenta, blue against yellow. When the tired side goes quiet, that balance tips the other way, so you see the complementary color: red becomes cyan-green, green becomes pink-magenta, blue becomes yellow. The ghost is not on the wall. It is your own eye still catching up.
Stare hard at a bright red phone screen for 30 seconds, then look at a white ceiling - a soft green-blue blob floats there and slides wherever your eyes go.
After you stare at a strong color, the ghost you see next is your tired eye tipping the balance toward its opposite, not anything really out there.
It is a clear, everyday proof that your eyes do not just record light - they adapt and add things that are not there, so what you see is partly built by you.
Tired color side goes quiet, the opposite side takes over - so red echoes cyan, green echoes magenta, blue echoes yellow.
Learn the idea and practice English at the same time.