As I'm taking you through in my process in detail on this work - here's a snapshot of how I've broken down the picture. Each part of the picture has been separated out into its own colour layers and named (as it's easy to lose track).
Now with everything isolated it's time to start painting all of the shading and details on top of each of these 'flat' layers, knowing that, for instance while I'm painting the shading and reflections on Nurse P's dress I don't have to worry about accidentally overpainting the background or any other part of her in the scene.
I'll concentrate on the base scene of Nurse P & Nikki like this before I add additional layers of 'spice' in the form of extra heavy rubber items and more delicious devices.