As the ball gets closer to the light source the effect of the light should be stronger – lighter highlights and darker shadows with the effect of the secondary light source less visible.
One question I have been asked a lot concerns the choice of colours. The very basic shading would be by simply using white for the highlights and black for the shadows. In my opinion the result usually looks dead and cold. I prefer warmer light and my favorite shadow colours are dark purple or dark brown (with adjusted alpha). That way you get warmer tones and more lively colours.
Then again the scene you are trying to influences the colours of your light and shadows. E.g. an underwater scene would look very odd with brown or red shadows – you would choose a white/ blue/ green colour-scheme. A summer afternoon would have a light yellow/ gold/ dark brown or deep red palette – while a winter’s day would have white/ blueish grey and black.
Here are a few examples of the affect colours have on the shading of an object.