Going back to the very original post on this, about the Dapol Sentinel. I note the model is black.
Since it was ordered by the GSR, it will certainly have been grey if THEY painted it, unless it was delivered by Sentinel in black and left that way for a while. However, there is another possibility.
Often, on other railways, contraptions like this copuld end up with a carriage livery, or something similar. At least one old picture of one of these quite new - or clean - appears to chow a shiny black chassis, but a slightly greyer colour above, though no lining.
I just wonder if it could have been the dark purple lake colour above (as on Downpatrick's coach 836)? Certainly, the number was applied in carriage-like shaded style.