We could wear any black dress (or skirt/blouse). 4 out of 5 sororities wore black, the fifth wore white. When I was a rho chi, I did informational pop-ins during pref (y'know, reminding PNMs where to go to sign their pref cards, things like that), and I must say I was impressed when I walked into the room of the sorority that wore white... they really stood out.
One year, our rush chair was adamant that we had to wear dresses or skirts. Guess who showed up in a black pant suit.
Edit: When I was a PX, we were asked to dress up on pref night, and we were told that we could wear anything EXCEPT black or white.