When painting snow, let cool colors predominate. As with regular watercolor, let the white of the paper stand in for the majority of your snow-covered surface. But remember that the shadows aren’t just blue. Don’t use a single pencil to create the shadow effects or they can end up looking boring. Look for a variety of blues to capture the shadows and the subject. Note that the shadows are generally sharper close to their subjects and become lighter and more diffuse as they fall away from them. Beginning with the lightest colors, you can suggest the rounded shapes of body shadows on the surface itself and use your sharper, darker accents to depict the shadows of trees and brush.
Here I laid in a neutral-colored background to suggest the snow clouds, using watercolor and a variegated mixture of Ultramarine Blue, Cobalt Blue and Burnt Sienna. I encouraged cloud shapes to develop by adding more or less water to my wash. When that was completely dry, I used white to dot the paper here and there to suggest flakes.
Blowing snow can be suggested with directional strokes blended judiciously with a damp brush. In this sample, I painted a simple house shape and a gray winter sky behind it with watercolor, then used dampened watercolor pencils and crayons with strong diagonal strokes to suggest the blowing snow.