These rules can be used in any medium from watercolor to oil paints.

Primary colors:

Those that you cannot mix other colors to create

Traditional: Red, yellow, and blue

Secondary colors:

Those you create by mixing primary colors together

Traditional: Orange, green, and purple

Tertiary colors:

Those you create by mixing primary and secondary colors together

Traditional: Yellow-green, yellow-orange, blue-green, blue-purple, red-purple, red-orange