In the retina, there are two different types of light-detecting cells: rods and cones. The rods are used for seeing in low-light conditions, and there’s only one kind of those. The cones are a different story. There are three kinds of cone cells that roughly correspond to the colors red, green and blue.
From the TED-Ed Lesson How we see color - Colm Kelleher
Animation by TED-Ed