Recently our class with Ed Praybe focused on painting a wide range of hues using only 3 colors – a ‘primary triad’ of red, yellow and blue. As you may remember, I’ve used such severely limited palettes many times, but only in oil paints so far. It was fun to try it with watercolors.
We did a test painting, blending the three colors before applying them to the paper. Our homework was to paint three versions of a still life setup: one monochrome to analyze the value (light and dark) structure of the scene; a second in which we mixed the colors before applying them to the paper; and a third in which we applied single color washes; let them dry completely, and then painted additional single color washes on top. This last technique creates new colors as the several layers combine optically.
Test case, pre-mixng yellow, red & blue before painting. 7″ x 5″. 
The set-up for the series of paintings. Note how the water in the glass distorts the shape of the apple behind. Neat. 
1st version, a value study using only ultramarine blue. 5″ x7″. 
2d version, pre-mixing the primary colors before applying. 5″ x 7″. 
3rd version, applying successive washes of single colors to produce different colors. 5″ x 7″.