What is Color

What Is Color?
Color is the byproduct of the spectrum of light, as it is reflected or absorbed, as received by the human eye and processed by the human brain. It's also a great design element!

Here's a surface level overview of how it all works:

The world is full of light. Visible light is made of seven wavelength groups. These are the colors you see in a rainbow: red, orange, yellow, green, blue, indigo, and violet—the Mr. ROY G. BIV you might have been introduced to in elementary school science. The reddish colors are the long wavelengths. The greenish colors are the mid-size wavelengths. The bluish color are the short wavelengths.

When light hits objects, some of the wavelengths are absorbed and some are reflected, depending on the materials in the object. The reflected wavelengths are what we perceive as the object's color.
Our eyes are the input channels, if you will, for this light. One portion of the eye is called the retina and it contains four types of light sensors. First are the rods, which record brightness and darkness and from which we "see" a sort of coarse sketch of the world. Next are three types of cones, each one optimized to absorb a different spectrum range of visible light. One set of cones absorbs long wavelengths, the reds. Another absorbs mid-size wavelengths, the greens. The third absorbs short wavelengths, the blues. Together, these rods and cones gather the information that our brain then processes into one combined image.
If you think this stuff is cool, Kimball's Biology Page's vision section explains the process in more detail and has a great scanning electron micrograph image of real rods and cones.
What this all means for the designer is that color is a function of light and biology—which means that no two people see color exactly the same. It also means that reproduced color can be described, defined, and modeled through a variety of mathematical and visual lenses called color spaces. Combine these two factors and you can quickly see how color—and its theory and use—can quickly take on the tone of a religious war.
The goal is to understand that color isn't an exact science and your job is to use it in the best way for your specific application.