Human eyes have three types of light-sensitive cells that respond to colors. These cells are called cones. Each type of cone responds to one of the primary colors red, green, or blue. One type of cone tells your brain when a light is green. A different type of cone tells your brain when a light is red.

But how do you see the yellow light in a traffic signal? Your brain combines two primary colors in order to see certain colors. For example, your brain combines green and red in order to see yellow.