I think that if your build is earlier than the 1750s or if you consider the shipowner to be wealthy and prestige concious, brass is appropriate. If you have brass, what would be even more likely would be a mix of iron and brass guns (brass in the minority), with the brass aft.
By the middle of the 18th century brass guns were very rare. Brass had advantages over iron but had become prohibitively expensive by the end of the 17th century for general naval use when compared to iron. Metallurgical advances had eliminated most of the disadvantages of iron by the end of the 18th century.
Brass was much much more resistant to corrosion than iron. It was definitely more attractive than iron, but more importantly, it had a significant advantage over iron in a materials characteristic known as 'fracture toughness', the sensitivity to defects from casting or corrosion. As a result, brass guns were much less likely to burst than their iron counterparts (probably another basis for presitige... keeping the 'top brass' the farthest from those unreliable, cheap iron guns). Although brass was always more expensive, even during the 16th and early 17th century, brass was still economical due to the fact that it could be recast into new cannon, whereas the iron guns had to be discarded at the end of their relatively short service lives. But during the 17th century iron guns got better and cheaper. By the middle of the 18th century, brass became reserved for prestige or special purpose pieces, of generally small caliber (4 pdr. or 6 pdr. pieces used on yachts, poop decks or small 'frontier' vessels where longevity was required - 18th century Spanish vessels in the Pacific for example). By the 19th century metallurgical advances in England with iron allowed shorter and lighter gun patterns, eliminating most of the technical advantages of brass.
Also, the popularity of brass diminished as naval doctrine focused on increased rates of fire. Brass retained more heat than iron and had poorer high temperature properties; the rate of fire was restricted by the brass gun's material properties, whereas with iron guns, rate of fire was restricted only by the crew's ability to feed and service the gun.
For both materials, guns were 'brown', not black. Unattended, in salt air, brass guns turned a medium brown with a 'purplish' hue as mentioned above. But don't go overboard on the purple, it shouldn't look purple, it should just cool off the brown a little. The effects of powder residue are to create darker spots and streaks where handled, so the guns should have a subtle, mottled appearance. But since brass was a prestige item and crews were large on men-of-war and that we all know that an idle mind is the tool of the devil... I'd use a bright finish on naval brass guns. For merchant ships and privateers.. brown and mottled.
Iron guns were polished and heated, then covered with what one observer described as a 'brown varnish'. An 1825 recipe for coating ordnance called for 40 lbs. of verdigris (greenish blue acetate of copper), 4 lbs. of 'Grant's black ground in oil', 3 lbs. of red lead, linseed oil and turpentine. If you can't find these ingredients, I would use a dark brown like burnt umber, maybe mixed with a little black and a touch of red for iron guns.