Exhaust staining on aircraft.

On dark colored aircraft the light stains are more visible (grey, light brown and even white)
On light colored aircraft dark staines are more visible (black and dark brown.)

i like to use a tamiya paint called "smoke". it's a translucent sort of gray. excellent for exhaust marks!

I've used Tamiya X-19 Smoke as a base for some time for exhaust staining followed by some gray, black or brown ground pastel chalk. Recently I picked up some eyeliner swabs from the make-up department of the local drug store and have been using them to apply the chalks without using the X-19 Smoke. The results have been pretty good, just make sure this is the absolute last step in finishing the model done AFTER your dull coat.

The exhaust stains colors will vary from aircraft to aircraft as they are a result of the combustion of the mixture (fuel to air ratio in the cylinder). A black stain indicates a very rich mixture whle a near white stain indicates a very lean (and hot) mixture. Ideally, if the engine is well maintained, the stain will be a light gray (slightly lean mixture) with a few streaks of black near the exhaust opening (that comes during shut down)