I pretty much suck at coloring...and backgrounds...and well, anything other than line art. LOL.
I think your coloring's been quite good, tbh. It seems to me like you just need to color a bit more and get comfortable with it like you are with your line art and you'd have coloring down easy. Your line art is pretty enviably good already. :) I'm so messy with my line art and I just have such difficulty smoothing out my lines, lol.