What is the difference between priming and underpainting?

While priming prepares the canvas ready to receive paint, underpainting is a first layer of paint applied after priming. Traditionally a lean layer with a reduced palette, it’s a tonal rendering of the eventual painting that serves as a base for the full colours and layers that follow. Underpainting helps you work out your composition, and when toning a ground it is helpful to not have to work on a white surface. Some artists also find it useful because it’s easier to get started on a painting without the distraction of a bright white surface.

Artist have traditionally used a range of palettes for grounds. For example, the method of imprimatura uses earth tones, such as umbers and siennas, while grisaille uses grey paints. Verdaccio underpainting is done using grey-greens and olives.