18 karat gold is around 10% copper. If copper is oxidized by chlorine to copper (II) chloride anhydrous, the result is a brown color...which, when combined with yellow gold, would probably look red-ish.
If something like this really is what happened, I hope at the very least it shuts up the snide nitwits who mock anyone for asking whether it's safe to perform a given activity while wearing an expensive watch.
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