Once you're used the sphere art worksheets, I suggest painting a page or two of spheres in your sketchbook. You may find it easier to draw the basic elements (use a lid or mug to draw the circle) before you start painting. If you use a watercolor pencil, the lines will 'dissolve' as you paint.
Use different colors to paint the spheres, to reinforce the fact that it's the values or tones that create the illusion of three dimensions, not the color with which you're painting. And paint versions with different values for the background too, as this influences the values you use for the sphere.
