hey pirate,
actually it is the girls in the sorority that wear the black dress. the PNM's can dress how they want but I would say wear something you would were to church or an afternoon wedding.
but here is my funny story about rush: I of course had no idea what to expect at rush or pref or any thing. I wore a black dress with a scarf and a red sweater. I walked into the room and all the sisters were wearing black and I was afraid that I should have worn something different