Just try experimenting! I'm also lacking in this aspect, but by doing it more we can be better. I'm glad that I can help. I'll try to see if there's any video/tutorials that can teach more about coloring in general, but honestly any kind of tutorials are worth watching, the art style might be different (realistic-anime-semi realism) but the basics are the same.I've been trying to do what you mentioned but I think I lack the knowledge of what colour to use and when/where to use it!
Oh and one thing that help my art tremendously, try using different kind of brushes and see which brush looks better, my art improved a lot because of it.
Now I have at least different brush for different situations (coloring, lineart, sketching, shading, effects).
