Osprey’s New Vanguard series includes my favorite references for Brit WW1 Arty:
British Artillery 1914-19, Heavy Artillery
British Artillery 1914-19, Field Army Artillery
Camo really came into being in WW1; but early in the war, many (most?) of the British artillery was gray. I tend to use Tamiya XF53 Neutral Gray. Here is my 9.45 in Trench Mortar from Varga Scale Models in gray.
It then trended to something more Khaki or “Service Color”. The Tamiya XF49 Khaki works well for the Khaki. Here is my Roden BL 8-inch howitzer with a base color of Khaki.
Service Color is browner - I’ve used Tamiya XF90 - it looks about right. Here is my Vargas Scale Models 6-inch Newton Trench Mortar in Service Color.
With the airplane and balloons playing a more prominent role in recon and target acquisition, camo became critical and artists got involved. There were lots of schemes, and some very bright colors. The exact colors remain much debated. I did my 9.2 Howitzer in a three color scheme documented by a British gunner / artist who colorized a photo of his gun. I used Tamiya XF68 for the green, Real Colors RC031 for the brown, and Tamiya XF60 for the yellow. I ended up toning down the green with a filter. I sealed this with Future before using an extra-fine Sharpie for the black interline. Once weathered and given a dust coat, I was pretty happy. This is my Resicast 9.2-Inch Howitzer. It took 2nd place at the 2021 Nationals in the fixed artillery category.
Bottom line is that there are a lot of opinions on the colors and not many facts. It gives the modeler a lot of leeway. Pick up the books, look at the photos and have fun. I hope this helps.