When you start a painting, how often are you using a white canvas? This video offers an alternative. There are a number of reasons for using a toned canvas, though the easiest explanation is time savings. If you're looking to create a rendered image featuring a full value scale, you might as well start in the middle. In the video I use a warm tan color, though it's worth noting that gray works just as well.
In the vocabulary of painting "value" refers to illumination. Light areas are high in value, dark areas are low in value. This video explores value sketching - in which a painter foregoes the line-drawing phase and skips straight to tone. If you've never done this sort of sketching before, it's a fantastic exercise!
From my experience, beginners are often taught to think in terms of line because pencils are so cheap and accessable. Working in value requires an artist to more carefully consider form and mass, which is a valuable way to think. Normally sketching in this way would require messy paints or charcoal, but working digitally makes using tone and value just as easy as lines.