Thanks for your observations Jonathan
I know that from my mainland experience, that if you look along a freight from the steam era up to the early eighties, all the wagons fade into much the same sort of colour, brownish brake dust, with fading from the sun(!) and wash from the rain.
You may know that all the wagons in the train started off different colours, but look at the train in 'long view' and all the train fades into a brownish/greyish colour.