Personally I've been struggling with coloring because there seems to be SO many different techniques to do it. And it compounded the problem because every artist I really like seems to do it completely differently.
Some artists say you should paint in value only (black and white) then add color. Marco Bucci I think is one of the more outspoken voices against that because to his point colors that begin to work off of each other create their own magic that coloring value simply will never get. And I've tried both and I honestly can't say which technique I actually even like best.
If for example you're doing the value based approach, it's usually the best route to do a gradient map to give everything a baseline, then you're using Overlay, Multiply and Normal layers together in different opacities to finish the job. If you're doing the what I might call the "painterly" approach, then a lot of these artists are painting the entire piece on usually just one layer, mistakes be damned and they consider mistakes to play in the finished piece.
One technique that I've used and really liked is once your overall piece is done and you're adding shadows, do a complete multiple layer over the entire canvas in one color and set it to like 40% opacity. Add a layer mask and then just start painting in shadows in huge sweeps. It starts to look "right" REALLY fast and you can change your shadow color from cool to warm to see how it affects the final result.