Found this drawing of mine and thought you might enjoy how it was done. I used Photoshop CS6 and a Wacom Bamboo.
Step #1:
This is a bit messy but it was an experiment and good enough for me :) The lines have to be in a seperate layer to make the coloring process as easy as possible. Yeah, I like slightly weird faces.
Step #2:
Each color should be in a seperate layer. The first layer containing the lines should be positioned above all of them, this will keep the colors in the background so they won't show on the lines. I find it easier to fill a whole layer with a color and then use a layer mask to spread it — because if for any reason you should dislike the color you've chosen, you then can correct it in one simple step by just filling another one into the layer.
Step #3:
Choose some colors for contouring and highlighting. I used a mauve and yellow-greyish color. Again in two seperate layers, above the previous coloring layers.
Step #4:
Your layers should look like these. For the contouring layer you can use "Linear Burn". For the highlights "Linear Dodge".
To finish it off, I played with a few curves to correct the colors, to make them pop out a bit more.
