In traffic, in that condition, they were never anything other than plain grey, though a small few were plain black in the late 50s. No coloured anything.
The RPSI used two liveries of their own; neither authentic. First their “own” livery of black with UTA-esque red rods and NCC-like red-backed numberplate, then the fictitious “South Eastern Railway” (of England) lined green for the 1978 film contract.
Finally, following a poll of members, it ended up as it is now in its correct GSWR / GSR / CIE grey.