Another bunch of hopefully useful advices. I will probably collect it all, make some nice pics and share it as a tut, but don't have a clue when. Probably on my vacation in August.
Wait, I was supposed to get some rest then Oh, well.
Don't be afraid of experimenting! Use overlay and multiply layers over the one you're working on, with different colors to see how it affects the look of your coloring. Use even dodge and burn sometimes, with different ranges (Highlights, Midtones, Shadows etc), but don't overuse it and don't use it as a main tool of your shading. Rather to highlight some element (works well with metal parts) or to add contrast. Use photo filters. Use adjustment layers. Photoshop is a powerful tool! You can be surprised of the result you wouldn't think of on your own. If you're afraid of spoiling your precious drawing, duplicate layers before experimenting or make a snapshot (the icon in the middle on the bottom of History window in PS. Saves a current