i am doing a UNIMOG 4x4 for a UN GB. i presume the exhaust pipe and muffler would be a steel color and not rusted up like i would do for a WW2 truck. i have been told the enfine would be metallic color as in unpainted. i have a 1/24 UNIMOG fire truck in the mail.
I would say probably a heat discolored stainless steel. I remember something like that on HMMWVs
Hello!
I searched the web and here are three ads by people selling used Unimog parts. Should give you the idea how they get old:
www.unimog-community.de/…/biete-auspuff-t76736.html
kleinanzeigen.ebay.de/…/93281247-276-3958
You could also make a picture search in Google looking for “Unimog Auspuff” (german for exhaust)
Hope it helps, have a nice day
Paweł
I do exhausts and other dirty steel by first painting with flat medium gray (I typically just use my gray primer) and then dry brush on some Testors steel.
hey stik as i posted to you on the 113 post. you are younger than i am. i was out before HUMVEEs
thanks for all the suggestions and the web search idea. i think i will do the mufler steel with a bit of eye shadow coloration and the exhaust pipe stell with a hint of redddish brown. of course than i’ll thorw mud and dust on it and no one will see anything anyway. maybe i’ll do a straight off the boat still pretty on the dock look.
when i start my INIMOG fire truck i’ll walk over to the fire station if i have some questions even if the kit is german and i am in the us.
Yeah, you have a bit of a headstart on me on the path to the pearly gates 
have a look at Testors “Jet Exhaust” for a good color to use with their steel color on the auspuff (I forgot how much I loved some German words). BTW, I did not get to play on HMMWVs until well after I left active duty in the late 80s and joined the Guard shortly thereafter. Although that is getting further and further behind me now…
Guten Fahrt
painted the exhaust pipe MMA steel, washed it it thin tamiya hull red and washed that with a weak oil paint raw umber. looks nice bit of a shine but not gloss.
OOPS all the rest was for the GB post, sorry.