Well, let's see. Rim Guard is beet juice, kind of a reddish color. Then there is likely no telling how clean the rim was when the fluid was put in there nor how long it has been gaining the delicious flavors and colors of rubber & rust.
BTW: It's a Schrader valve, sometimes known as an American valve, invented by August Schrader in 1891. Not to be confused with the Presta valve or European valve.
Well, I guess it was a dark reddish, like I say a brown color. Didn’t up and stink, but not like fresh beet juice, either.
Maybe it is Rimguard then.