Coloring is hard. It is one of the things which there are no shortcut of learning it. If you don't follow proper steps on doing it, the end result would be horrible, or a fucking nightmare if you try to fix it.
Imho, it is twice harder than drawing especially when you do it in order to achieve into a certain level of quality. One thing that you need to concern about coloring is color balance. You have to color it so that it looks natural, and lively.
It is a tedious work and you have to make sure that you're doing it right the right way.