Way back in the 2nd grade we all learn that the primary colors are red, yellow and blue. And while those are the primary colors for print and/or canvas, the primary colors we use for a computer or a television are red, GREEN, and blue. These are called additive colors. Additive because these colors are created once you add light to them. When two of the colors are mixed together they create an even brighter color like cyan, magenta, or yellow.
As the additive color adds (brightens) the subtractive colors (red, yellow and blue) take away light (darkens). The subtractive colors are used more for painting and printing. When two or more of the colors are mixed together, they create an even darker color.
Complementary colors are two colors that are opposite from each other on the color wheel. They are complementary to each other because when mixed together they will create a neutral (gray, black or white) shade. The two colors are obviously also appealing to the eye when shown together.
So there is my little tid-bit about color theory! Here is a helpful chart to be able to more visualize what I have explained!