The tuning of the engine will have an effect on the color of the smoke emitted from the exhaust, and thus the staining on the aircraft. Alison engines in particular usually have light colored stains. Merlin engines are more variable. The underlying surface also plays a part on the visible staining. In your case I would not only study the reference pic of this aircraft, but also other Merlin P-51s and determine the colors and patterns for exhaust staining.