I built a model of a Lancester some years ago that Missing Lynx were kind enough to post in the gallery at
The correct colour worried me for months. Taditionally British Armoured cars of this period are painted grey, in various shades from light grey to Battleship Grey.
I beleive that all amoured cars built in Britain probably started out grey.
Well, this was the traditional colour of the undercoat applied to all railway engines, bridges and ships before delivery. Clients then applied whatever livery they wished to paint over the undercoat.
I beleive that many early shots of armoured cars show this undercoat, in same way that many railway engines at this time were photographed like this as they left the works.
In an account by a veteran from the RNAS, I read years ago that the cars with Locker Lampson in Russia were painted a "greenish brown" colour.
At Duxford there are several pieces of WWI artillery in a similar "greeny brown" colour. I used a dark earth colour with a hint of green to arrive at a similar colour.
Good luck with your model.