Colors are the sensations we receive from various frequencies of light. Our eyes have special cells that respond to specific frequencies, so some may react to the frequencies of red light, others to green, and so on.
White light contains all the frequencies, mixed together; true darkness is the ultimate black, with no light reaching our eyes.
Color theory can get complicated, but there are two basic ways that images can be colored. First, you can paint or stain them. In this case, you apply a substance that *absorbs* many frequencies of light--for example, if you are painting a toy fire-truck red, the red paint is actually absorbing all the other frequencies of light, leaving only the red to reflect back to your eyes.
Second, you can illuminate the object with colored light, as is done in stage lighting, for instance. With the right light, you could make a fire-truck (or anything else) appear to be red, even though it was actually painted white. You are *adding* the specific frequencies that you want to show.
This matters is for those using these techniques: mixing colors for painters is different for lighting designers. Painters who mix colors tend toward a darker and darker blend the more colors they mix together--theoretically, you'd expect black, but in practice, it tends to be a very muddy dark brown. Lighting designers who mix different colors get results that tend more toward white light.
Hope this helps a bit!