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I mean, painted helmets were almost definitely a thing back in the day. Probably extremely rare but I'll bet my bottom dollar that some wealthy knights thought to themselves "How badass wouldn't I look if my helmet was bright red"
actually the opposite, paint inhibits rust but is much cheaper than polishing, which does the same. late medieval period a noble's set of armor would cost ~10% in materials and 90% in labor, with most of it being polishing work