Have you ever tried painting an entire painting with only three colors? You would be amazed at all the different colors you can create by mixing only three colors. Start with your primary colors: primary red magenta, primary yellow and primary blue cyan. Mix them together and you have your secondary colors: green, orange, purple. Then there is your tertiary colors and so on.
Grab yourself a sheet of watercolor paper and do some test color swatches. Choose any cool red, yellow and blue. Swash them in a circle, in a straight line, however you choose. Blend each color with the next. Next try this with a warm shade of red, yellow and blue. Then be daring and try this color mixing experiment with any three colors on your palette and see what you come up with. There is a whole range of colors right there on your palette that I bet you didn't know were there!
Mix 'em up! After completing your experiment of colors, choose a set of three colors and now create a painting using only those three colors.