Pvt Snuffy, you didn't say what period you were interested in, here is a list for the colonial period:-
British gun carriages/limbers: Light blue-gray until the 2nd Boer War (1899-1902) when gun carriages and limbers began to be painted some shade of khaki or olive drab.
British naval brigade gun carriages: Dark gray or black. You could prime with black & then dry brush a dark and/or medium gray along with steel.
French gun carriages/limbers: Bottle green (a deep green with olive tones).
Russian gun carriages/limbers: Apple green with unpainted steel or bronze barrels.
Italian gun carriages/limbers: medium blue with black metal parts
Japanese gun carriages/limbers: Light gray with unpainted steel barrels.
German/Prussian gun carriages/limbers: Medium blue-gray carriage with unpainted steel barrels.
Egyptian gun carriages/limbers: Krupp guns would be medium blue-gray with steel barrels. The Egyptians also used bronze La Hitte rifled guns from France so it would be reasonable to paint them French artillery green. They also had some small bronze mountain guns from France that would presumably be green as well.
Mahdist gun carriages: Left-over Napoleonic muzzle-loaders used by the Mahdists could be a weathered wood color which is a silvery gray.
US Army gun carriages/limbers: US gun carriages were a mustard to olive green color. Artillery units were issued yellow ocher paint and black paint and instructed to mix them together to achieve the desired shade; the final color depended on how much black they used. The target shade was very close to modern olive drab.
US Navy gun carriages/limbers: The steel carriage and barrel were painted black as per Navy Regulations of the time . The wood in the wheels were stained brown and then varnished. They didn't start painting everything gray. in the Navy until the Span-Am war.
finally dips his toe into the world of Aeronef.
