RLM 81 being green is actually not a mystery. As Mr. Michael Ullman found out a few years back, there never was a green RLM 83. Instead it was a dark blue colour intended for the use on naval planes in the Mediterranean as a substitute for RLM 72.
Thus all the dark green and brown colours we see on late war german aircraft are just variations of RLM 81.
That also shows why Focke Wulf, on internal documents, called RLM 81 dark green, Dornier called it olive brown and Messerschmitt called it brown violet.
All the best and keep up the good work
A. Timper