Could not help but respond to the idea that the Navy used grey for guns. Check the naval ordinance manuals, esp 1866. They do have a grey paint on their list, but not for guns. Naval ordnance (iron) is painted black and that did not change until the the 20th century. When Bannerman purchased all the surplus Dahlgren Boat Guns fron the Navy, they were in storage and all wood was painted black (bronze guns-black furniture). The barrels are a greenish grey from tarnish. Look at the store pictures.

The Great White Fleet even had black barrels. Look at the photos of the ships in the Spanish American War. Grey naval guns have only been with us about 100 years. I would not discount that when mixing the paints based on the formulations in the manuals in the 1860's, you might wind up with a dark grey or dark brown, but that was not the standard. Confederates used anything they had but the preference was to look military and black was the barrel color.