I don't know any on the web, but from what I know, using your pencil is very interesting... you can press down
harder on the page with your pencil in the darker area and then work outwards towards let's say the edge of the
drawing (let's say a shirt) by lessening the pressure you press down, so on one side it's darker than the
other... and this is only with a single pencil colour. When you get the hang of it you'll eventually want to mix in
two, three or four, five six or even seven colours together (most probably different shades of one colour or in come
cases blue purple, red orange... similar colours) to create a more dynamic effect. Check up toning on the net.... oh yeah... there's this really cool site this guy has and he's a portrait
artist. It's in Japanese, but by looking at his style of drawing, you'll get what I mean ;)
http://www005.upp.so-net.ne.jp/PG-Jun/index.htm
Happy colouring ^_^