It might illustrate why I am so skeptical.

When I was a teen, I had a chemistry set (no surprise there )

My brother who is 11 yrs younger than I, was a little boy. These sets were
very common in those days, but not so much now because of safety rules we now have. There were many little
experiments in oxidation/reduction reactions.

One would take a clear liquid of an iron compound, and add a few drops of
another clear solution, like sodium thiosulfate, and bingo, get a fancy precipitate. Same for copper, and other metallic ions. Same with acetic acid.

Now, 45+yrs later, I don't remember them all. But they were MAGIC for my brother! My parents used this fascination to control his behavior..you know...if you behave..she will do the magic for you!

I think the moisture from sweat reacts with the heavy metals (the ones I bolded in the post above) and changes their state to a precipitate, which is non soluble and colored. The vinegar is in there to provide the acidic format in
case the person does not have acidic skin.