For paint, the primary colors are red, blue, and yellow. (Primary
colors are colors that you can mix together to make any other colors.)
For this kind of colors, the complementary color is what you get when
you mix the other two together. (For example, the complementary color
of red paint is what you get when you mix blue paint with yellow paint
- green paint.)
Also, complementary colors are the pairs of colors that look the
most noticeable. (This isn't a very scientific definition, but it
works.) For example, blue and orange are complementary colors, and they
stand out against each other much more than say, blue and green do.
Complementary colors are discussed in more detail in our
"Complementary Colors" answer. You can get to that one by going to the
Light and Sound: Properties of Light section of our answer list.
(published on 10/22/2007)