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Start handing out blue bracelets.
Or ask him into his face if his colleagues will beat him up, too, if he loses his bracelet.
At a protest many years ago in Germany, two undercover cops were beaten up pretty badly by other cops. They had a "safeword", but apparently the other cops were already in a frenzy and didn't stop.
A beating frenzy
Forces that regularly do plainclothes often have a "color of the day" for this reason.