In: Physics
We perceive color using cells in the back of our eyes called cone cells. There are three different types of cone cells, and each roughly responds to red, green, or blue light, the three additive primary colors. For example, when you look at a red image, the red cones are stimulated and tell your brain that the object is red. The different cone cell types work together for you to see other colors, which are mixtures of the three primary colors. For example, if you look at a purple image, which is a mix of red and blue, both the red and blue cones are stimulated. When all three primary colors are mixed, the result is white. All three types of cones are stimulated to see white light.
If you look at one color for very long, the cone cells can become fatigued and temporarily do not respond. For example, if you stare at a yellow object, and immediately look at a white area afterwards, you will see an afterimage that is the same size and shape, but is a blue-green, or cyan, color. This is because your eyes use the red, green, and blue cone cells to perceive white light, but because the yellow combination of that cone cells are fatigued, you do not see red. You are left seeing with only your remaining cone cells. After several seconds, your fatigued cones will recover; the afterimage will fade away and white colors will appear normal.