Getting used to looking at one color can lead you to perceive an entirely different color.
Stare at the eye of the red parrot while you slowly count to 20, then immediately look at one spot in the empty birdcage. The faint, ghostly image of a blue-green bird should appear in the cage.
Try the same thing with the green cardinal, and a faint magenta bird should appear.
The ghostly birds are called afterimages. An after image is an image that stays with you even after you have stopped looking at something. As you stare at the red bird, light-sensitive cells at the back of your eyes become less responsive to red light. When you shift your gaze to the birdcage, your visual system “subtracts” red light from the white light that’s being reflected from the white background. White light minus red light is blue-green light. That’s why the afterimage of the parrot is blue-green. A similar thing happens when you stare at the green bird, but this time your eyes become less responsive to green light. White light minus green light is magenta light, so you see the afterimage as a magenta cardinal.