With a very diluted mix of OMS (Odorless Mineral Spirits) and some burnt sienna paint, I brush on a tone to get rid of the white of the canvas. You can use ochre, or red. Rub with a paper towel or rag to spread the tone evenly. You can see that the pencil sketch still shows through. The purpose of the tone is to give you a midtone color, which you can then judge your colors against, more easily than white. Also if you miss a spot here or there, you still have a midtone underneath.