Just as good and evil can sometimes be Color Coded For Your Convenience, you can also sometimes tell them apart when villains dress in outfits that are overall far skimpier and sexier than what the heroes wear.

Although it's not that common nowadays, this likely started as a form of Getting Crap Past The Radar. "It's okay if they dress immorally. They're the bad guys! (wink)"

This trope doesn't require the good guys to dress up like prudes, nor that they never put on sexy clothing (because, you know, Fanservice). It's just that their regular outfits are significantly less skimpy than what the villains wear (and the heroes can dress like that with an Evil Costume Switch).

The exact way an outfit is skimpier can vary. It could be anything from more revealing necklines to being outright Stripperific.

This often overlaps with Evil Is Sexy, except this doesn't require the villain to actually be sexy, just dress sexy (there is a difference, as sometimes the skimpy dressed villain can just be a skank or just not attractive). Furthermore that trope often has the villains dress just as sensibly as the heroes, or the heroes to dress just as sexy as the villains (most Super Hero comics). Neither of those are this trope.

Compare Good Eyes Evil Eyes.

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Rolling Examples:

• The picture comes from Yaiba, where the moon empress◊ dresses like a Playboy Bunny (she's apparently an anthropomorphic rabbit possessing a human), while the heroes dress sensibly.
• The Dark Queen of Battletoads. Even though the toads were technically naked, it's not really skimpy due to them being obviously anthropomorphic.
• Lunar The Silver Star. Luna wears a peasant dress at first, then practically nothing after her (apparent) Face Heel Turn.
• Compare Rinoa and Ultimecia in Final Fantasy VIII. Rinoa may be prettier, but evil gets a dress where the neckline ends at the crotch. And then it's slit up to the crotch.
• Dr. Frank N Furter who often dresses in nothing more than a corset, fishnets, heels, and short shorts (or panties...). When others "succumb" to his power, they start dressing this way.
• Harribel, Grimmjow, Loly, Lillinet from Bleach.
• In Tarot Witch Of The Black Rose, the dark witch's outfit are even skimpier than those of her good sister, and that's really saying something.
• The Tin Man recent sf fi original film with Neal Mcdon(something) and Zoey Deschanel, the evil sis had some costumes.
• Pricess Aura's metal bikini with shoulders & cape from Flash Gordon.
• I-NO of Guilty Gear is... improperly dressed. Works for "That Man", who appears to be The Man Behind The Man.
• The Mirror Universe in Star Trek