I think exhaust color is specific to the engine and even prop on a given engine. I ran an MVVS 1.5D recently and exhaust residue was very dark. The engine had a certain bark to it. This is the second one I've had. I had one 12 years ago which ran the same and had the same dark exhaust. The MVVS 2.5D was similar. I just ran a Norvel .074 conversion and the exhaust was was virtually clear. It probably needs some more run in time, but I don't expect much change.