I don't think you're going to find any one tutorial for 'coloring'...there are a lot of sub-topics within that, like picking a palette, shading/highlighting, coloring effects/elements, atmospheric coloring/shading, texturing with color, etc. etc. etc. And honestly, depending on whether you paint or do cel-shading, the way you learn each of those sub-topics is going to be different...
I'd suggest searching for each coloring issue individually as they come up (for example, if you're stuck on how to pick colors for a nighttime scene, or how to shade someone's sweater, search 'nighttime palettes' or 'shading clothes'). You'll find lots more help that way.
To start you off, here's a big thread here on Tapas covering a wide range of topics. Hopefully it'll give you some ideas on techniques you'd like to try: