A nifty trick that made this turn out so good for me was the separation of the shading and breaking it down into simple parts. First, I selected the entire sketch in an outline on a new layer, picked a grayscale-ish colour, and filled the entire thing in, then set the layer to multiplay so that it didn't overlap the lines. I made another layer after this, setting it to multiply as well, and took the basic colour palette for the thing, and made the "flat" underlying colours for it. A new normal layer, and I added in the highlights of light using the exact same colour pallet as before using a gentle approach with an airbrush. Another normal layer, with an airbrush, I hid the other layers save for the linework, and gently used an airbrush to add the shadow contrast in. Then you un-hide all the layers and see how things look. And if you're happy, just merge them all together and off you go! If you wanna try getting neat shading, give that a try and see how your work turns up. So far this seems to be a pretty solid method for adding colour to characters done in line art.