I actually just manually choose the colors for highlights for cel colored images. For my airbrush images I use this method: http://lilirulu.tumblr.com/post/147949559286/so-here-is-some-info-on-how-i-render-things-now
And for what colors I use: http://lilirulu.tumblr.com/post/153350473116/how-do-you-choose-your-colors-like-in-the-hair-of