From wall paint to clothing, the world is obssessed with color. Color is the most basic decoration, color coordination is viewed as an essential skill of fashion, and the rainbow enchants adults and children alike. How is it that color exists? The answer lies in light. In the absence of light, colors don't really exist -- after all, you would have no way of knowing a wall was purple if you didn't have light to see it with. In fact, "color" is a quality carried by the light itself as it reflects of certain surfaces!

How can light be different colors? After all, light is light! The answer lies in the frequency ν (nu) and wavelength λ (lambda) of light, which you can think of as how densely compacted a stream of light is. These qualities are inversely correlated according to a factor of c, or the speed of light: .

http://commons.wikimedia.org/wiki/File:Wavelength_indeterminacy.jpg

As it turns out, different wavelengths correspond to a variety of kinds of light on something called the electromagnetic spectrum, ranging from radio waves to gamma rays, with visible light somewhere in between. On the radio wave side, these wavelengths can measure into kilometers, but visible light takes up a comparably tiny portion measured entirely in micrometers (µm).

http://science.hq.nasa.gov/kids/imagers/ems/visible.html

You may object that light is white and the sun doesn't shine purple! This is correct: in white light, all wavelengths of visible light travel together. However, certain objects absorb some wavelengths and reflect others. Leaves are green, for instance, because they absorb everything but green, leaving the green spectrum light to bounce around and eventually hit your eye. Thus, we can see all the colors of the rainbow -- and more!