Not exactly sure how it was with military vehicles, but, MOWOG green is only party accurate. My understanding is that while the engines were painted green, in reality they just kept topping up the vats with green - so, there is no standard green, just a variety of shades of which Moss picked one.
BTW in MG world, a MOWOG is also a mythical three foot high creature with four inch long fingers suitable for undoing the nuts under the dashboard - and various other tasks.