This is a detail of a drawing I did for a sign for a Comic Convention and a label. This is a good look at what that formula I was mentioning on the other color girl (the one wearing jeans) is all about. By separating the linework onto another level you can just wipe a giant brush across it and not worry about the edges, as it stays only on the lines. This is useful when you want to do a fade on a place like her lip lines, in this case from red to black. The other neat thing is how sloppy you can be with the paint as it's all underneath the lines, so you can be pretty fast with it.
It's amazing what a change it is to just take the lines from black to brown.