Once that solid color is filled in, you can either leave the outlines in black or change their colors to a complementing one with the
Ink Bottle Tool for a softer, more blended outline. Sometimes Ill use a darker tone, sometimes a lighter one; it depends on how much I want the line to stand out from the color, and what kind of look I want from the overall piece.
The most difficult part of the shading is dealing with overlapping lines; when all of the lines were black, they blended into each other--but when working with many colors, I find that sometimes I have to tweak a few vertices so that my colored lines dont intrude into the wrong areas.