This is mainly how I'd do cel shading, and I need to say that this is the method I like to use, so it's definitely not the only or the best method out that's there. Plus, it's by no mean a standalone method, as to get a better result it often pays to combine cel shading with "normal" painting. But for a simple cel-shaded anime look, this is the basis of the method I use.

This tutorial was done in Photoshop CS2, but should work with other software that has blending modes.

Frankly, I don't expect this to be all that helpful, but since there's no one way to colour a picture I find it difficult to write a meaningful colouring tutorial. Hence I'm simply showing some of my basic methods, if anyone's interested.