Depends upon the era and the arm.
Naval Gatlings had a black paint/coating on the barrels; the other parts were brass adn bronze; and the mount was supertructure color.
QF guns under 5 pounders were laso blaxk-coated, ecepting the working bits. Above that dimension, barrels were, often, in either deck or superstructure color.
To throw a wrinkle in that, 5" 50s were a mix of black painted steel and natural bronze & brass. At least until about 1940, then they went to deck or superstructure color (except the secondaries on BB-35 Texas, those stayed black).
20mm had barrels that were in a black phosphate (Parkerizing) finish, the frame and mounts were in superstructure color unless the Camo Measure directed Deck color. Magazines were a semi-gloss black. Shileds were typically superstructure color. Shoulder braces were black or superstructure paint with brown leather padding. Straps were a pale grey buff.
40mm were a whole mi of colors. The recievers were in deck and supertructure color. The springs were black finished, barrels were charcoal grey phosphate (although some were in deck color). The flash cones on the muzzle were all sorts of colors--you have to check references.
3"25 & 3"50 were pretty much painted all over, other than a few working parts in natural metal. The late-war 3"50 with auto loaders had dark grey phosphate barrels (ecept when they were painted; post war examples were never painted) The radar was a mih of grey and balck. The autoloader was red.
5" 38 open mounts were painted all over. The internal parts for 5" in gunhouses were pale grey to white, with natural metal "bits" inside, paited to Measure outside.
Modern Cal..50 MGs are Parkerized; there will be a charcoal grey ofer a gunmetal sort of color. That Phosphate runs from near black to a medium grey. If the arm wasa coated in cosmoline and wrapped in Kraft paper, the phospate finish gets a green tinge (this is the classic look of the M-1 Garand).
25mm chain guns are a mi of phosphate finishes. all tucked into a superstructure pain mount
