Those are both great examples of why a character might try to throw anti-troll paste (tar) at trolls.
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A red cape doesn't mean anything different than a black cape to a bull, and sucking venom from a snakebite isn't a thing outside of movies. It won't hurt, but it won't do much good either. One myth that will cause damage is putting ice on a pit viper bite though, so remember to avoid doing that.