Sue... I was suprised at the temp change in one hour..handling the flexible bag with 5gal of that temp water is asking for trouble...I used black, but it would be a good experiment just to see the difference with the yellow....the theory is the longer wavelength red end of the electromagnetic spectrum is the heat end, and the black should absorb all the visible frequencies, while the yellow pigment reflects and not absorbs yellow ...yellow is close to the red end, so some available heat energy is reflected....the effeciency may not matter if you are just looking for a way to heat water without electricity


another one... a MRE hot beverage bag, or easier to find home made pouch from a vacuum food saver bag (double sealed vertically slighty over 1/2 the width)makes a durable water transport bag that folds up pretty small