Some people have asked me questions like 'can you teach me how to colour the way you do, or do you have any tips about colouring/shading in general', and while I'm not exactly learned enough to give proper advice, I can certainly do a step-by-step explaining the way I colour using the most basic tools and techniques available.
Done in Sai with default brushes. Well, the bar 1 em setting is custom, but you're probably going to get the same effect using Flat Rough or some other similar flat brush.
The important thing is to know how to use the dropper tool to ease into colours nicely and paint using flat shading efficiently before attempting any blending, and of course, having an understanding of the way light falls over objetcts. If you're having trouble with shadows, I suggest practicing with simplistic shapes and objects.
also, I'm lazy so I constantly flatten and work over my layers to avoid having to juggle them. If you have the patience to manage multiple layers for a neater lineart/more manageable shading then go right ahead, but I find that painting over the top of everything works best for me. (I keep backgrounds separate from character images of course, and things like effects and ambiant lighting on their own layers)