Hi!
Oil coming out of the silencer should always be clean...never gray!
Black it could be if you run a pipe and use 20% castor oil or if you race your engine in sport Q-500 racing with a Webra GT .40 running on 80/20 FAI fuel (castor oil).
But never gray or black if you run synthetic oil.
A gray colour always means that there is metal particles in the exhaust oil....No good!

Regards!
Jan K
Sweden