Why do we see things in colour?

The human eye and brain together translate light into colour. Light receptors within the eye transmit messages to the brain, which produces the familiar sensations of colour.

Colour is not inherent in objects. Rather, the surface of an object reflects some colours and absorbs all the others. We perceive only the reflected colours.

For example, red is not "in" an apple. The surface of the apple is reflecting the wavelengths we see as red and absorbing all the rest. An object appears white when it reflects all wavelengths and black when it absorbs them all.

Sources:
http://www.pantone.com/pages/pantone/Pantone.aspx?pg=19357&ca=29