Could load it up in inkscape or gimp and shade those areas, myself I like gray with black lines, that way I can tell when the blade veers off the black line it shows up agaisnt the gray some like red against the black black.
Doing this watch for open areas or it colors erverything so if there is a gap in a line and you hit color and you don't want it colored it fills it in, somebody else thats more skilled in using these programs could explain it better.