Gino, that looks remarkably like weathered Khaki Green No.3 on your top tube...Have we seen a photo of the whole machine and of the frame number ? Is that an original stamping ?
In my experience of ex-WD bikes, it's not that unusual for owners in the 1950s or so to have replaced a damaged / bashed / blown up bike or major component with cheap old ex-WD stock and then simply transferred over the number from their logbook...far less trouble and expense but a puzzle for us, sixty years down the line...
It's a mystery!
It does look like faded green in the photo, I'll take a proper look in the daylight to confirm. There is also some light blue in their!
Thanks for all your input & comments,
G.