From my research on the NG DRGW locos it was hit or miss on the green boilers as not every loco got one.
I used to try to find a dated photo of loco in question, it is challenging as sometimes you had to decide base on
B & W photos. You could spot color differences provided loco wasn't too dirty!
I personally took the green ones as the exception , that and they faded to an (unpleasant to my eye) shade of green
Good luck on the research.
Dave