I think I've cracked it! Finally managed to figure out how to colour my pencil sketches without doing that dodgy 'erase background' thing that leaves white bits everywhere. All I had to do was set the layer to Multiply didn't I?? Silly person. Now I can colour directly underneath my sketch and its all so easy! So excuse the bad shadowing that I've done below (its just a rough on an already scanned sketch I had handy)... but this is kinda what I was struggling to achieve. Ahh yes. Maybe more work will happen now.