Hi there Derek

You're right, there's not much info out there at all! From what I've been able to find the fuel drums come in two sizes: British 45 gallon and US 55 gallon. If its the US version they would have been painted in Olive Drab, the British ones were generally first painted in British Army Green (sort of mixture of Khaki and dark green-ish) ie, same colour as the green part of the army vehicle camouflage, but nearly all the photos I've found of the desert campaign seem to show all stores of this size painted in the British Desert Yellow (again the same as the vehicles) which makes sense as that's much easier to hide in the desert!
There were no different colours for the different contents - fuel, water, oil etc, the contents were stenciled onto the lid or sides. Some units might have painted the lids in a different colour but this seems to be the exception.
Your best bet for a diorama would be to give the drums a coat of the darker green for a base coat and then a top coat of desert yellow leaving a few bits of green undercoat showing through here and there, either through wear and tear or just slap-dash painting (they weren't all artists like us! )

Robin.