This is a slight insight to how I shade. Sometimes...
If you want a full on tutorial with screenshots and heaps of nice helpful tips, I'll make it.
If not, then... this is it!

Anyways, I choose a base colour, which is almost NEVER the top right corner, it's always a bit lower than that so it's easier to highlight...

The slight shading is normal shading, you colour as much as you need shaded with it, normally I shade less, but this is an example of a red ball in EXTREMELY EXTREME lighting, okay?

The shading is also added, I always use at least two colourse to shade.

Dark shading is for bright lights like fires or explosions. Mind, I add other stuff when lights are like that, but this is for SHADING, not LIGHTING. Perhaps I'll post how I light things if you request.

Use this sparingly! Dark shading in large amounts can ruin a picture!

Then the highlight, a tiny little squiggle on top. This is where the light is, it lightens up the colour. As you see, using the top corner red, I had to go pink for the highlight, that's odd. You'd like to have the highest highlight up in that corner, not the base colour.

Rebound light. TINY squiggle. And I mean TINY. This is a tiny little faint rebound of light on the other side of the object, it's not the focal point of light! Beware!

So. there is an unsmudged ball there, and for a better look a smudged one. I smudge a lot. Also, sometimes I use the burn/dodge tool, but this little insight hting was mostly meant for a special Paint user *cough mercury-angel cough*

Hope you liked this!