In fine arts, there are three primary colors: red, blue, and yellow. They are called primary colors because they cannot be created by mixing other colors. Primary colors form the basis for color theory or color mixing, as using these three colors it's possible to mix most other colors.
A primary color can be any of the red, blue, or yellow pigments available to a painter. Each combination will give you a different result, and that's part of what makes color mixing with paints so interesting. You can also use the primaries used in printing (magazines, newspapers etc.) which are magenta, cyan, and yellow (plus black), but limiting yourself to these means you never explore the rich potential of paint color mixing and the subtle differences between pigments.