Not a colour expert, but I'm pretty much the same as rebel, do the whole pic in white, letting the black underneath show through for shading etc, and getting in as amuch detail as possible. Tidy up, and get rid of unwanted overspray with black, and then use it to define areas that need it, Then start adding colour with transparents, lightest to darkest, remembering that the colour of the layer below will affect the layer you add (eg, putting red over white = pink etc), so compensating where needed, then lastly add highlights.