Well, not many may know this, but when i work on the chapters every week since i am a Linux user i have to use Gimp. But now i prefer it over photoshop. So here are a few tutorials.
Lineart tutorial by pheona
Coloring & Cell and soft shade by pheona
Coloring eyes by gazproffesser
Basic Coloring by Marco Lamberto
Another Basic Coloring by blackspyder
All in one
gimp clean and colorization
Covers Cleaning and Coloring pictures are in French so might be a bit hard to understand all the body text is in English though.
Please post more tutorials that you find relating to gimp.