Here’s a chart of watercolor pigment combinations. The purpose of the exercise is to see how colors interact when they’re superimposed, rather than mixed wet. After one color swatch is painted and allowed to dry, a second color is laid over it.
The starting colors, on the diagonal from lower left to upper right, are: Winsor yellow, Winsor red, permanent alizarin crimson, ultramarine blue, cerulean blue, and Winsor green. Each possible mixture sits at the intersection of a horizontal row and a vertical column.
The arrow, for example, shows a combination of permanent alizarin crimson and ultramarine blue. Farther to the right, the mixture of Winsor green and alizarin crimson makes a beautiful gray whose component colors are still visible.