I have a beginners photoshop tutorial in my gallery, but for other stuff I suggest you look at my tutorial folder in my favorites. It's filled with the most useful tutorials I've found. Many are about coloring. By coloring I'm assuming you mean blending is what you're having difficulties with? and not cell shading which is very simple and almost self explanatory.
Once you get the feel of it, the best thing to do is loose sketches. Then try drawing different types of lines using the pressure sensitivity as much as possible. I don't really understand what you mean about your difficulty colouring, as that's a pretty basic thing? Fill the area with colour then shade it. Look at some tutorials, I can't give too much help with that as it's a personal thing.
I havent the slightest clue what mypaint is lol. The advice I can give is to customize your tablet buttons through wacom tablet properties, and most importantly zoom in when you are doing more delicate work like lines and such, so you have more control.
Mostly practice, practice, practice. On YouTube there are a ton of exercise for tablet users and artists in general. And on ConceptArt.org they have what's called C.O.W. which is creature of the week. They give you a one-line creature description and some rules and you make your own version of that category. It's a good way to get out of your comfort zone and not get hung up on trying to think of something to practice when you just want to get right to practicing.
Well, same applies with the Youtube thing for lots of tutorial and watching some examples of people's drawing techniques. There's also several artists here on DA who have LiveStream sessions, so you can watch them while they're working.