A shot of my desktop as I was working on the quick and dirty coloring of  shep's pic. The basic steps are:

1) Use the B&W sketch as the background. Add an alpha channel so the layers can be manipulated.

2) Add a transparent layer. Do the color airbrushing on this.

3) Duplicate the B&W sketch layer and put it on top. Set it to "Darken Only", and the lineart will overlay the airbrushing for the finished effect.

You can probably skip a step and just put the sketch on top and make it "darken only", but then you get the invisible layer checkerboarding, which is annoying. :)

Hope this helps! :)