All the ones I have ever seen were brass, and I'm sure after a couple days in the field they rather quickly started tarnishing after being out in the weather, so you didn't have to worry about the enemy seeing the sunlight glint off your fire extinguisher. However, I'm also sure that since GIs are GIs, the fire bottles also got painted every shade of green in the book, if and when their vehicles were repainted in the field.
I volunteer every year on the USS Torsk submarine museum - a couple vets of the boat said brass sure is pretty, and whenever the crew had to do routine maintenance, they were told "If it's brass, polish it. If it's already painted, paint it again." Well, polishing is harder than painting over, so gradually almost every brass fitting, air line, switch box cover, etc. would become painted instead of the natural brass! The same probably happened in the Army field of operations as well, but as far as I know, all Pyrene bottles were issued brass.