I haven't done a commission in a while and I can't seem to get confident to colour in my work. Are there any tips/guides for colouring and shading?

Keep in mind if you're going to send me advice or tutorials, I own a tablet and I draw on GIMP.

Here's the piece I'm working on right now. It's a commission piece in its line art stage.

Well I don't use GIMP so I don't know the details of what your program can do. But I would use a top right light source, so the character is looking up at the sun. Do you have a palette you need/want to use? Or a particular style of colouring? Cell, painterly etc... I almost invariably use cell shading, and in this case I think muted warm colours with a grain filter of some form would work well.