AFAIK it's supposed to be the natural color. I typically take my replacement tanks, wipe them down with solvent, and spray clear over the entire thing. It keeps the bare metal look, but won't rust or oxidize too much.
Did you see the size of that chicken?! -Dirty Steve
the original finish was applied to the steel sheets before it was formed into a tank. that finish is called ni-terne. it is what the repro tanks have. you can coat it with boeshield t-9 to preserve the finish.