I always prefer to work on a toned canvas instead of white and I wanted to share what I think is the easiest method for toning canvas. The way I do it now is that I take a small amount of ivory black on a cotton rag and add a touch of Gamsol and then I simple rub it gently with circular motions into the canvas. It's very simple and the best part is that it dries very quickly. I also like the texture that the rag makes on the surface to give it a little character so it's not a perfectly flat tone.
You can tone your canvas any color that you like, I prefer to use ivory black to make a cool gray, but you could also use burnt umber, raw umber and burnt sienna as some other options. I recommend using an earth tone because they're neutral, they dry fairly quickly and they're very stable pigments.
Next time you paint, test out some canvas tones!