In the top and middle photo, the dark camouflage colour appears to be a shade match and possibly a colour match for the slouch hats so I would think that either a dark green or khaki brown. The second colour is slightly darker than the shirts so a lighter green or brown for that.
The camouflage on the bottom photo has a very dark colour against a very light colour, perhaps a very dark green for the dark area and a cream or sand colour for the light area.
The above is just my guess; I am by no means an expert on camouflage or colour interpretation from B&W photos.
Whilst on the subject of camoflaged guns this American gun may be of interest. Apart from the camo it was the very last Allied gun to fire during WW1. It seems that the gun commander's watch was slow so they were still firing a little after no side was called.
The painting depicts a 6 inch howitzer of the 1st Cdn. Heavy Battery in action near Thiepval on July 16, 1916. It is by Lt. Kenneth Forbes, a war artist who served with the Machine Gun Corps in France. (Courtesy of the Canadian War Museum)
The artwork collections of the Imperial War Museum and the Australian War Memorial are searchable by subject on-line, and images of many of the paintings are posted on their sites. The "Great War in a Different Light" site has an extensive section devoted to war art. One of my favourite print sources is Purnell's "History of the First World War" which was published as a series of 128 heavily illustrated magazines in the late 60's.
I suspect that the colours shown in the painting are probably applicable to most B.E.F. formations, camouflage being the general responsibility of the Royal Engineers Special Works Park. These R.E. detachments were eventually provided to each of the armies in the B.E.F. Stephen Bull has published some information on R.E. camouflage paints.
BTW, Centurion, thanks for the "heads-up" on "A Sailor of Austria" - it was a very enjoyable read!
Stephen Bull identified common R.E. colours as light and dark Brunswick green, ochre, and burnt umber. I received this info second hand - I think the source may be Bull's book in the Brassey History of Uniforms series "World War One: British Army" (1998).
