I've noticed that some people, including adam wilder & mig jiminez,sometimes paint the cannon on their german tanks a dark grey color and everything else in a standard camo. Is their a specific reason for this? I know the cannons were replaceable, but did the spares leave the factory in this color?

Yes, that was the protective lacquer they applied to the barrels. Sometimes they were in such a hurry to return the tanks to active duty they didn't have time to paint the barrels to match the camo.

Barrels were factory-painted in heat-resistant lacquer which could be either grey or dark yellow. It is evident from colour photos that some barrels painted grey were fitted to vehicles produced long after Dunkelgelb replaced Dunkelgrau as the basic colour for vehicles and that this applied to various types.

I have not seen dark yellow barrels on grey vehicles so logically the introduction of the yellow lacquer was probably after the 1943 adoption of Dunkelgelb.