If it's not in the Boyd book, I doubt you'll get a definitive answer, short of deep digging in the IRRS library, and even then that info may not have survived. I have the book and can check but like other similar books I say just a colour mentioned in passing.
Most loco greens of that period were darkish, tending towards olive green. A default green that the makers slapped on rather than railway officials going through colour cards for their preferred shade.