My understanding of the way the warthog works is that it separates hydrogen and oxygen from water. Then it recombines them in the internal combustion engine for explosive force resulting in water vapor for exhaust. That would mean those red cans on the back are full of water. Nice to know the UNSC is environmentally conscious while blowing up halo rings. ;-)
Oh, and I kinda like the red cans.
This was always my understanding also-- I personally thought the idea was pretty cool! And I've never had a problem with the visual of the fuel canisters being present on the Hogs; it is technically more visible noise for the design, but it doesn't go overboard into overdesigned territory IMO and also is a practical element.
Only real headscratcher for me is why they'd be carrying water in red jerry cans- my understanding for the real-world precedent for red gas cans is to make sure there's a clear visual warning element concerning the volatility/toxicity of the contents (Gasoline). I can't think of a great reason for the UNSC to continue that archaic tradition when they're only toting water around- the cans do
throw a wrench into the presumably low-profile OD design of the Hog.
I'm nitpicking; in any event, I can think of about a thousand more things that the H4-5 art direction did that I find insanely more ridiculous, busy, and confusing than the cans on the Hogs.