Hi Peter, a good question, although to some extent it also depends upon which period you are referring to.
- Napoleonic field, siege & garrison artillery - there is little factual evidence but it is generally thought that wood was dark grey & metal was black. Contemporary drawings also show blues & buff - buff is thought to indicate natural unpainted wood.
- Victorian field, siege & garrison artillery - this era is blessed with better documentation. Zinc oxide paint found favour over lead oxide paint but did not wholly supersede its use, in overly simple terms the oxide was mixed with linseed oil in one form or another & driers & sometimes pigment were added, formulas can be found in various references as can the times allowed to clean & paint each nature of ordnance & its carriage. Some specific colour applications are known, one reference states - White paint for lettering etc - Lead colour for carriages for home service - Stone colour for powder cases etc - Red for trench carts, pontoons, inside boat magazine, leather etc - Black for iron work.
Naval - 1750s - Carriages were mainly red. Mortar beds (on Bomb vessels) land service grey.
Naval - Napoleonic - Almost always invariably the colour was red ochre simply because that was the cheapest paint. Apart from red some other examples known of such as white in officers cabins, green for Lord High Admiral, & scarlet & gold for certain yachts.
Naval - Victorian - I have fewer references pertaining to this subject area & can not state with any certainty what the practice was.
Adrian