Sorry, TJ .... I doubt that practices in the US Army (presumably) during the latter part of the 20th century (.... unless you are a
lot older than I assume ....
....) would have much application to British Empire ordnance and ammunition of the late 19th century. I think you are simply seeing the age patina and/or light effects on the cast iron body of the shell, with most of any original paint being long gone.
According to the 1887 Treatise on Ammunition, all studded RML projectiles of this sort were painted black, with the projecting studs (usually composed of gunmetal or zinc) left unpainted. If you look at Redbadge's "3rd pic" (fifth post in this thread) it appears that remnants of the original black paint remain on the iron case around the gunmetal stud, where it has been protected from wear and abrasion .....
Redbadge, did you ever get measurements of this shell before they removed it? I am wondering if the "2 5" on the bottom might signify 2.5", which was the bore size of both 7-pdr and 9-pdr RML guns (The 9-pdr had somewhat "beefier" tube walls and fired a heavier shell with a larger propellant charge .... The rate of twist of the two types of guns also differed, so that the spacing/offset of the studs differed on the two types of shell. Accordingly, a 9-pdr shell could not be loaded in a 7-pdr tube.)