An artist friend in California sent an email with this idea. Historically, charcoal and pencils have commonly been used for under-drawings on canvas. The problem is, graphite can show through transparent paints, and neither charcoal nor graphite deal with the issues raised by color. One solution is to do the under-drawing in color, using regular school chalk. If you don’t like the way an area is drawn, just erase it with a wet rag and re-do it. When you have the color under-drawing perfect, paint right on top of it.
Chalk is made from calcium carbonate, a filler already used in many commercial oil paints and mediums. It helps with consistency and stability.