Thank you Rallye Driver for your excellent answer.
I never thought that the Earth Brown/Dark Green combination was designed for lower altitudes as well as for when aircraft were dispersed on the ground but as it was developed in the late 1930's when aircraft flew lower that makes a lot of sense.
Also makes sense to retain it for larger night bombing aircraft - as almost any choice of colors would appear as tones of grey by night the greater visual threat would be from air attack while on the ground for which the Brown/Green scheme would be more effective.