When you say "layers," do you mean like PS layers? So do you keep one layer as a sketch and then do a gray one, and then colors on top? I have worked in PS a little bit but not enough to develop any sort of technique.
yeah 1 sketch (dropped to 30 % opacity, on top of the whole stack), 1 bg, 1 painting, (on top of bg) and 1 for some post process stuff//blur. PS is pretty fun when you get to know the ins and outs of it. id suggest learning it just by playing around with it. theres tons of tutorials here on DA