I just posted this in answer to a question about painting with soft pastel. As it touches on underpainting in this medium I thought I'd add it here.
This is a portrait of me done by my teacher Professor Lockard as a class demo. This is just with the three primaries and it is a value study, nothing about local color. Thus all that yellow in my beard is not to suggest "blonde", but the grey of age(sigh).
Note that the value range is narrow and pigment is lightly applied. This is a finished underpainting ready for the finishing colors which will extend the value range to the lightest lights and the darkest darks and address local color issues like skin tone
"Three's so little room for error."--Elwell