Yes, a bit like modelling UTA goods - the greatest part of the "livery" by far was just pure filth, faded nondescript paint, and a heavy covering of brake dust.
I have a big interest in the railways of Majorca. Steam ended there in the early 1960s, and it is thought that some of the steam locomotives were NEVER repainted from delivery in the 1910s. That gives forty years of weathering. Cleaning locos was not their strong point either, and it was probably just as well given the faded horrors underneath.
Suffice to say, that despite detailed perusal of several (good quality) photographs of a locomotive I knew to be officially green, taken in 1963 or so, there was not one solitary trace anywhere of anything green, or black; from wheels to cab to side tanks to dome to chimney, the whole engine (brass nameplate included) was just pure weathered soot, smoke, brake dust, and track dust. An accurate scale model of one made in fluorescent dayglow pink would not have to be repainted at all - just weathered until not one detail of its original state was visible!
Many old wagons were like that too.