To actually color you should use the filling/gradient tool or the brush.
The brush should be set to, Hardness: 0%
Opacity: 10% - 20%
Flow: 100%
Now, to create shading you simply lower the size and color the same place, if you want it to be a bit darker just change the color it self to dark, oh and don't forget to change the brush to multiply.
Also, you'll have to use a lot of layers, like if you want to color something, you can simply choose it with the pen tool right click on what you chose then press on "Make Selection" and every time you color it won't go out side, you'll color only what you selected, now if you want to shade what's inside the selection you'll have to change the opacity.
(http://img689.imageshack.us/img689/8886/77187737.png)
Step one, choose the pen tool
(http://img715.imageshack.us/img715/6306/stpone.png)
Step two, make a path
(http://img862.imageshack.us/img862/4181/stp2.png)
Step three, right click and press on make selection
(http://img641.imageshack.us/img641/1223/stp3.png)
Step four, choose the brush and change the hardness to 0
(http://img194.imageshack.us/img194/2868/stp4.png)
Step five, change the settings of the brush to: Soft Light
Opacity: 20
(http://img607.imageshack.us/img607/7728/stp5.png)
Step six, color.
I hope it makes it easier, it'll take you some time to make the path but it makes your life a lot more easier, well for me it does...
Thanks and yeah Each color I used is a different layer and the background layer (base layer) was the actual sketch and I still suck but whatever man it's cool :(
and I love the color you did with the hair!
I actually did this one drawing ( (http://fc05.deviantart.net/fs70/f/2011/357/c/f/karkatxterezi_by_nickybobby14-d4dyi5c.png)
) without any layers (or anything for that matter, I just drew it like it were a normal picture) and it took me like a week or two, but I think it turned out pretty alright, I was proud of it. Still as I've said before, I am more of a traditional artist, I like pencils and paint better. Plus ever since my computer exploded I don't have photoshop on it anymore and I can't find the disk to reinstall it ;_;
You'll get better, just keep practicing. :)