The Australian Army did extensive testing (published as "Survival Water in Australia's Arid Lands", by Kavenagh) in the 80's. Transpiration means breathing. The stomata on the leaves open up when distressed to evaporate water to cool the leaves. The stomata also close in the dark, as inside a dark bag. This reduces yield significantly.

I was told by the former OC of the Army Survival School in Darwin that opaque bags also break down due to the heat generated and can generate phosgene-like offgas which of course goes into the hot water inside the bag. Why take the risk? Clear 44-gallon drum liners are the issued item here, 2 carried behind the kidney-pad on most army rucks.