My suggestion is, as Walt said, to use layers.
Speciffically this way(I've been doing things with this in a VERY basic state and it's pretty useful):
- Layer 1: outlines and flat colours. This may be the perfect point for a basic CS start. As Step 2 will save you A LOT of time.
- Layer 2: Throw some light at it. Depending on the style you want, you'd have to place the light source in one or another place. In this case, I suggest you to put some light balloons on "chest/boobs", "shoulders", "head"(and part of the face if there's no hair bangs around), and upper part of her arms. Then I'd go putting one of these ballons in her left leg(femur and knee area), since it's forward and nothing covers it.
Now, blend layers(erasing the 3D model, of course). When the "lightest" balloon hits one of the silouete colour(the darkest), you can change them into the second(flat colour), so this will save you A LOT of time.
Basically wityh these 2 layers, you've got three colours in each area. You can make the next step as I do OR using a third layer, which basically is adding another balloon/line between the lightest and the flat colour, so you can have the intermediate colour between lightest and flat.
Basically, this process could save you a lot of time.
Have a nice day!
I swear there was something cool here!!