Print the color mixing triangle out and trace it onto a sheet of watercolor paper or, if your printer has waterproof ink in it, print it directly onto a sheet of watercolor paper.
Paint the three primary colors in the corners of the triangle as shown -- red, yellow, and blue. Then mix them together to create the secondary colors (orange, green, and purple) as show in this finished, painted triangle. For step-by-step instructions, see How to Paint a Color Theory Triangle.
The first color triangle is attributed to the French painter Delacroix. A notebook of his dating from around 1834 has drawing of a triangle with the three primaries written in as rouge (red) at the top, jaune (yellow) on the left, and bleu (blue) on the right, plus added the three secondaries as orange, violet, and vert (green). Delacroix adapted the triangle from a color wheel in an oil painting handbook by J.F.L. Mérimée, a painter he knew.1
What You Need to Know About Color Theory for Painting
Color Mixing Tips
Color Mixing Quiz
1. Colour and Culture by John Gage. Thames and Hudson, London, 1993. Page 173.