Select a round brush, turn OFF all presets. Set to 500px, 100% hardness, 100% flow, 100% opacity.
From the palette layer, select a dark version of the hue of your choice, a medium version of the same hue, and highlight color (different hue from the base color, at maximum saturation). Paint a circle of each color to the right on the palette layer, and lock the layer so you don’t accidentally paint on it.
Change brush to 1000px, 5% hardness, 5% flow, 100% opacity.
Hold down the Alt key while moving eyedropper over darkest color. Release Alt key to select.
You can add colors to the swatch palette to use in future.
Rough in the sphere:
On a new layer, reactivate your circle selection from the last exercise, or make a new circle-shaped selection using ellipse select tool.
With long, curved strokes, draw in the shade on the bottom right of the sphere with the darkest color. Use medium color to rough in the rest of the shape.
Add the highlight color using a smaller brush (500px). Use white to further lighten the highlight area, and to add the reflection area.
On a new layer beneath the sphere, brush in the shadow in black.
Blend the colors:
The colors aren't perfectly blended yet. Reactivate the selection if needed. To blend, Alt+select a color in the area between two colors, and overpaint with that. Continue until entire sphere is blended.
Zoom in (Z or Ctrl+ +) to blend, while keeping an eye on the Navigator to see how your changes affect the overall image.
Can't change a rubric once you've started using it.