All of our senses give us vital information about our surroundings, but the one we rely on most is vision. Barring vision-impairing conditions, we walk (or drive, or ride) safely through the world, recognize friends (and enemies), read, write, and otherwise learn what's happening by the power of sight. The sense organ for vision is an exquisitely evolved biological instrument for turning light into the brain's language of electrical signals. The retina contains nerve cells as well as a layer of 120 million rods and cones, receptor cells that respond to light. Some react primarily to red, some to green, and some to blue light. Because of the brain's ability to organize information about the relative intensity of these three primary colors' essence, mixing them from such a simple palette, we can distinguish millions of colors.

Source: Dana Foundation