I haven't been able to find any evidence that the Brits painted their tanks green. I have no idea why everybody paints their models that colour, other than the fact that the ones in museums are usually painted in this post-war colour.
As far as I can tell, the early tanks were painted in multi-coloured camouflage patterns (some of them very bizarre), and later when they realised that all that work was covered in brown mud in about ten minutes they just started painting them the colour of mud in the first place.
I'm sorry but I have no knowledge of Vallejo. Do they do a nice muddy brown?