okay for pre-ban.
not okay for post ban (must not be "flash hider" and would have to be permanently attached)
As for the color, as far as I know all the YHM is the same color you find on most any AR--looks grey in sunlight but more like charcoal in low light. The finish matches my pre ban Eagle Arms and old Bushmaster AR finish just fine.
As for noise, I've never noticed any difference other than maybe redirecting the sound more perpendicular to the barrel making it loud for the people standing left and right of you but about the same for the shooter. Never tried standing down range.