Rich
If I can recall they wore either a one piece overall jumpsuit a bit of light khaki color I think plus some had dark brown trousers with a dark leather jerkin. It is really hard to tell accurately as originals are faded and few color pictures exist ( I discount so called 'colorized' photos I wish they had left them in their original format of B&W ) I just looked up in my Schiffer books and the pictures they have of tankers 1916 shows them in the 1908 or 1914 standard infantry issued uniform, I actually have a picture of a tanker in that style uniform except he has on shorts in place of trousers and puttees. They originally formed part of The Machine Gun Corps.
The Osprey book British Army 1916-18 western front has them in the same color as the infantry of the period with the dark leather chain mask and goggles. I hope this helps you Rich
( Now you see why I like the Great War, after years of study I still don't have firm answers on uniforms, I guess I am rather picky or color blind LOL)
signed
'General Melchik'