An artist will use color to express a mood, the atmosphere or to stimulate tranquility. To understand color you must first understand there are only three true colors: red, yellow and blue. These are called primary colors, and with those colors you can make any other color except white. If you look at the color wheel and you start with yellow, and you put red into it, it becomes orange, yellow and blue make green, blue and red make violet, these we call secondary colors which are orange, green and violet. That is the primary color wheel, then you can go one step further to develop the tertiary color, that is mixing the two secondary colors, but by doing so this will probably end up with a muddy color.

The reason you buy more than the three basic colors is convenience, and the other colors offer better hues and a creative color palette.