Brooke rifles and smoothbore cannon were made of cast iron. They were manufactured by Tredagar Foundry and the Selma Naval Ordnance Works during the civil war. When banded, the bands were made from wrought iron while the guns were cast iron. Cast iron guns, both large and field guns, were painted black as the material could rust in the weather. The book "The Big Guns" by Olmstead, Stark and Tucker, published by Museum Restoration Service in 1997 covers Brooke rifles and smoothbores in chapter 9, pages 125 to 125 has pictures in this chapter which show some Brooke guns with what appears to be a dull finish in a light brown shade of color. Hard to tell since the pictures are not in color. The exterior of the Brooke guns were not smoothed at the foundry and remained rough in finish. This rough finish and a modern day coating may contribute to the finish as they appear in the pictures. The book is silent about any coatings that may have been applied to the guns during or after the civil war.
Regards
Ron