The principle is very simple: Girls HATE showing up to a party wearing the same thing as other girls. So, as she walks by, point on something on her, then make eye contact and say:
“You know, I almost wore that EXACT SAME THING tonight. That would have been SO embarrassing.”
If she looks stunned and doesn’t laugh right away, ask her what color it is (Particularly in a dark club or if you have shades on). I always lean in and say “Is that blue, or is it green??” Kino escalation by touching whatever it is she’s wearing.
“Oh, it’s green? My dress/teddy/negligee/coonskin cap is blue, so it’s not that bad.”
Which naturally leads into… “Have you ever been at the same club/party/restaurant/coonskin cap with another girl in the same dress but different color? Is that less embarrassing?”
She should be amused and intrigued by this point.
– I Almost wore that Opener by Unknown Pua