This is quite extreme, the most layers I have used in this image is three per color. Each color has a base, highlight and shadow. Let's begin with the hair color. I first began by filling the hair region with the 'base' purple color. Now make sure this layer is below the outline, it will make things easier.
Some people say to use the magic wand.
This is against my beliefs entirely. By using the magic wand, you do not select the entire region, as the wand is confused by the anti-aliasing of your clean airbrush lines. It yields a much better result to color the region by hand using the paintbrush or airbrush with hard edged brushes. Once the base color is laid in, the highlight color is added. I use my tablet to improvise and edit the highlight shape on-screen, as I find it to be the easiest way.