When I did the old Monogram kit a few years ago, I mixed some light gray with various shades of brown to produce a color for exhaust stains, then applied at very low PSI through my airbrush. If I can find a picture I will share it.
Here are my pictures:
If I was to build another P-61, I would probably add some chipping and "fade" the black with some lightened black across some of the upper surfaces, pick out some panels like that.