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