I think Lt Mitchell who commanded the British tanks in the first tank vs tabk battle describe the Tanks as being painted a "muddy brown"
The great war channel recently put on a special at the Bovington tank museum where a man from the museum said the tanks were painted "dog poop brown". this special is mentioned in the research section on this site.
Is it not the case that Mk IVs were at first painted green and then later brown (leaving aside exact shades for a moment)? I have seen discussion elsewhere of "Service Colour" being used. This was definitely a green shade: wherever I saw it had the pigment recipe.
Bovington has a builder's model of the Mk VIII in a case. This is said to be painted in the genuine factory 1918 colour and to possibly be the only known genuine sample of that colour. It's brown, but exhibits greenish fading after 100 years - and its only ever been indoors. Excuse the glass reflection in the picture below: I couldn't eliminate it. Their MkII is definitely green, but that cannot be taken as gospel because she was later converted into a supply tank and then post-war into a replica MkI until the Museum was gifted the last surviving MkI.
Whippets were green: all of them AFAIK. But MkVs built at the same time were brown.
None of the tanks at Bovington show any signs of a red primer on their tracks where worn back to bare metal. Most have been painted black, which is almost certainly not original. I would be surprised if they were painted at all in service: what would be the point? They were made from armour plate, which is inherently rust-resistant (not rust proof) because of the alloy components. The track link displayed next to Little Willie is unpainted and has weathered to the grey-brown oxide typical of high-manganese steel.