I love painting shaded spheres of color! Often, when I'm tired in the evening but still want to do a little something in the studio, this is one of my favorite quiet, low-stress activities. This is an excellent exercise for practicing leaving the white of the paper. Remember that in traditional watercolor, there's no use of white paint; you either paint around or mask out the white of the paper to serve as the white in your painting. Painting shaded spheres is also a great way to play with color blending.
Step 1: Draw a circle lightly in pencil by tracing around a lid or using a circle template.
Step 2: Choose the position of your light source. Using a round brush and leaving the highlight of the sphere unpainted, lay in a light, flat wash of color. Here I used quinacridone magenta. Let dry.
Step 3: When dry, add another layer of the same color slightly smaller than the first.
Step 4: While Step 3 is still slightly damp, lay in another similiarly-hued color over the area you painted in the previous step. In the example, I used deep cadmiuim red. Let dry slightly.
Step 5: While Step 4 is still slightly damp and using the very tip of the brush, lay in a line of complimentary color on the edge opposite the light source. Complimentary colors are opposite each other on the color wheel so here I used sap green. Two complimentary colors mixed will yield a neutral color. Let dry slightly and proceed to Step 6 if desired.
Step 6: Add a small amount of dark color to bottom edge of sphere (where shadows would be the darkest.) Here I used Payne's grey.