Curiously, those things were technically "silver" (= unpainted) until, double-stacked for beet in the 1990s, when like everything else by then, they became brown.
As built, they had standard grey chassis, like any grey wagon. Earliest examples also had a tiny stencilled flying snail on the chassis. As life went on, both the grey chassis and unpainted corrugated body became a homogenous dirt colour. Only in the last few years before they were double-stacked, the chassis (only) of a small number of them was crudely painted brown, sometimes just a patch behind a painted number. While single height, none of the bodies were ever painted any colour.