Demo

I painted this little sketch to demonstrate the sort of effect you can get by layering with thin washes of paint (glazing). Each layer remains partially visible through subsequent layers, allowing the colors to mix optically.
In this case I was using Alizarin Crimson, Ultramarine Blue, Cadmium Yellow Deep, and white.

I began with a layer of green.
























Then purple in the shadows. Notice the swatches to the side showing the colors I'm using.
























More purple to define and deepen the shadows
























A wash of Alizarin Crimson
























Then a wash of Cad. Yellow
























Going over that with Cad. Yellow mixed with a bit of white























Adding a touch of Alizarin Crimson + white in select places
























Background

























There's still a lot more I could do to this, but I'll stop there. Hopefully this makes the underpainting/layering concept a little clearer for you guys.