How does the eye perceive colour?

There are two different kinds of photo receptor cells in your retina: rods and cones. The rods are not capable of perceiving color. They are only sensitive to variations in the intensity of light. Rod cells function only in dim light. Some animals have almost no color vision because their eyes are full of almost entirely rod receptors.

The cone receptors in our eyes are responsible for our ability to perceive color, but there are much fewer of these types of receptors in our eyes. As a matter of fact rods outnumber cones by a factor of about 20 to 1, except i n the retina's center (called the fovea). They are highly concentrated in this part of the eye. The cones require bright light in order to be activated and able to function.

Additive Color System

Your computer monitors uses three phosphors to emit beams of light in three different wavelengths that can combine to produce all the colors it can create. Each of these differently colored beams of light affects a specific conal receptor in your eye and therefore lets you see the color. Notice when you combine red and green beams of light, you see yellow, when you combine green and blue beams of light, you see cyan, when you combine blue and red beams of light, you see magenta. If you send each of the three beams of light with equal intensity, you'll see white. If you don't combine any of the beams of light you'll see black or an absence of light on your monitor. Red, Green and Blue are called the Additive Primaries.

Substractive Color System

You can also create color by allowing the reflective quality of objects to substract colors from the white light available in nature before it reaches your eyes. You don't see the colors that get absorbed, you only see the colors that are reflected. This is how color paints and pigments work. Printing uses this substractive system by using four colors Cyan, Magenta, Yellow and Black to create all of the colors you see in most magazines and printed materials. In printing these colors are know as the process colors. Black is added because artificial pigments are not capable of absorbing all colors except one, they are impure. If you were to combine the cyan, magenta and yellow pigments in full intensity they would produce a dark brown instead of a pure black as in the example above.