Ken
That’s absolutely correct. The thing never had a repaint in its entire life! Along with the 800 class trio, it’s the solitary engine that the GSR didn’t paint grey!
The exact shade of green is unknown, but if it were possible to research in England what the makers generally used, you’d be ok with that. The only thing the GSR did was to add a number plate, and since the engine was never repainted, it’s probable that the number plate was just unpainted metal too.
Those Peckett photos above would probably be correct, but in GSR days s heavy layer of weathering, almost obliterating the livery as in some photos above, would be more appropriate!