I have now completed the colouring for the character sketches. I have tried to maintain a traditional inked look, with a simpler colouring scheme (mostly flat colours with edge highlighting and light shadowing.
This is the first time I've done some major colouring in GIMP (GNU Image Manipulation Program), a free image editing tool that is pretty close to Photoshop. It's a great tool for what it does, and (hey!) it's free, so can't complain!