As with anything in model building, There's always the attempt to simulate reality... For the life of me, I can't seem to come up with a paint or a mixture of paints to simulate exhaust stains on a aircraft.. In this case I'm almost complet with a Bf-190 G6, I've started weathering and can't get that "thinned brown" further away from the center of exhaust stream to the "powdery black/grey" on the center of exhaust stream...... by making it suttle and realistic...

Do I accept my losses and airbrush black, or pastel the exhaust stream ..........

Have you tried using oils/pastels? AFAIK, they blend in well with the underlying color.

Oils and pastels works for me

Just a follow up on this post...
I had reciever emails from other modelers explainning to me their technique on exhaust stains....
It looks like the general idea is to thin heavealy a blend of paints and build up the stain accordingly.
In my case I used Tamiya's Flat Black/Red Brown mixture at a 50/50 blend, thinned heavely with alcohol.
Several passes with the airbrush along the wing root.
Later, after all weathering is done, A Dark Grey pastel will be brushes at the immediate exhausts for that powdery look.