Bonjour Greg!
You may already have consulted the following texts which have brief notes about the colour scheme applied to the B.E.2s of 50 Squadron, The Air Defence of Britain 1914-1918, by Cole and Cheesman and
British Aviation Squadron Markings of World War I, by Rogers, but, on the chance that you have not I will quote from both.
From The Air Defense of Britain 1914-1918 -
"Malcolm Christie was responsible for the first specialized colour scheme for night fighters. During August [1916] he black-doped all his B.E.s, explaining to the Dover Garrison Commander that this was partly for invisibility but mainly to prevent pilots from being blinded when using their wingtip landing flares. To help identify these sinister black aircraft as friendly during day training flights, white rings the same size as the outer blue of the national roundels were painted on the upper and lower wing surfaces. GHQ Home Forces approved the scheme and promulgated it for adoption by all 'black-winged night flying machines' on 29 August. Many months passed before it was appreciated that the white circles did much to destroy the desired invisibility."
A caption to the well known photograph of a B.E.2e standing on nose At Detling in 1917 featuring the skull and crossed bones with 50 below the cross point of the bones on the lower surface of the upper wing reads "The skull and crossbones device adopted by No.50 Squadron was a natural follow-up of the black colour scheme."
From British Aviation Squadron Markings of World War I -
"He [Christie] had all his aircraft doped over-all black. However, when they were airborne during the day they had no national markings and were at risk from friendly aircraft and unfriendly A.A. guns. Therefore, white rings were painted on wings and fuselage which destroyed any value of the black covering. The idea was accepted by H.Q. Home Forces, who promulgated it for adoption by "all night flying aircraft" on 29th August, 1916. Photographs show this scheme was not always used."
The same photograph is shown with the simple comment "Note the under-wing marking" ...
Do you have a copy of RAF BE2e, WINDSOCK DATAFILE 14 by Bruce? In the COLOURS AND MARKINGS column by Rimmell there are notes about the markings of the 50 Squadron BE2es and a line drawing of one circa 1917. Rimmell does write "The all-black BE2cs and 2es of No. 50 HDS at Dover are interesting in that they adopted a skull and crossbones motif on wings and fuselage although it should be noted that on the wing undersurfaces it was more common for the top of the skull to face the leading edge."
So, I would assume that when 50 Squadron was formed in May, 1916 the aircraft were not painted in the black scheme, that being adopted in August. At some point following that time the the skull with the crossed bones and 50 motif was added to some, if not all, of the aircraft.
Salut!
Kirk