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Unless it was added recently, the Japanese Wiki doesn't actually say what they are. My guess is that they're all weak to Purple. When fighting them, it seemed as though the people who were wielding purple weapons were dealing more damage. However, as I don't really have concrete proof of such a thing (I didn't do any testing of the matter), I cannot guarantee the validity of the statement.
"A shouted order to do something of dubious morality with an unpredictable outcome? Thweeet!" My FC is in my profile.