How do you get stains out of canvas?

A colour wash on canvas is quite simple.

Dilute your paint to the consistency of milk.

Use a wide brush and lay the canvas flat.

Start at one side, with a fully loaded brush and sweep it smoothly across the a canvas, moving down with each stroke to achieve a thin layer of paint over the entire canvas.

Practice on old canvas to get the "feel" for this technique before you start a new project.