EDIT: Sorry - completely missed the bit about doing it as a ceremonial gun. I have no idea on those. I'll keep the below in case you're interested.
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It could have been in a range of colour schemes as 1943 appears to have been a bit of a time of flux for paint schemes.
There are some good guides written by a chap called Mike Starmer. They come with paint chips so you can match the colours. It may be worth trying to pick them up before you strip the paint, as if you sand back the layers you may find out some of the history of which theatres your gun served in. The books are for vehicles rather than artillery, but it's probably a fair assumption that the same paint was used for both.
The main book is "British Army Colours & Disruptive Camouflage in the United Kingdom, France & NW Europe 1936-45"
There are special books for "Desert Colours : Alamein & After 1942-1943" and "Middle East Colours : Sicily and Italy 1943-1945"
Looking in those books, BS 987C became generally available around September 1942, with other instructions following through 1943for the use of SCC2 Brown as a basic colour, and SCC14 Blue-Black as the disruptive pattern. That would have been the European theatre, so it's assuming that the gun left the factory in this paint scheme, and that the same paint scheme was used for guns as vehicles.
I'd imagine someone on here should be able to confirm that - there's probably an MTP46 Pamphlet for artillery pieces or guns which may give you some more info.