A natural metal finish will tend to show less staining than a painted surface. WW2 aircraft camouflage paint was generally made with a matt or satin finish to avoid reflections. To get a matt finish the paint has a powder mixed into it which gives it a rough texture and this tends to collect dirt. The exhaust stain is a mixture of oil and other combustion byproducts like lead. Oil stains tend to be a dark red brown, but lead stains are very light. Sometimes planes have just dark staining and sometimes both light and dark. For instance F6Fs tend to have a pale centre to their exhaust stains with a dark border. Lancs have a lot of pale lead staining. You should always start by looking at as many WW2 photos of your plane as you can and examine the staining and other weathering. Be wary of modern warbird photos as those planes tend to be way more pampered than RL service aircraft.
I paint my stains pretty much the same way as Devil does, although I do use a mouse. I have a drawing pad, but have never really got on with it. I set my airbrush tool to say 15-30 pixels diameter, about 60% hardness, 50% density and 5-10% opacity. Then I make lots of strokes in the direction of the airflow building it up thicker near the exhaust but fading as it runs back along the fuselage and to the edges of the stain. On some aircraft the staining can run right back to the tail, you can often see faint discolouration running across rear fuselage markings. If the exhaust stain has both light and dark areas I might make a seperate layer for each. That way, if I decide I don't like one of the colours, it is easy to change it later.
