Once there was a Panhellenic BBQ before rush officially started, to which we all wore the Panhel t-shirt and any bottoms. Maybe other chapters coordinated, but we didn't, so skirt, shorts, capris, etc.
Round 1:Letters with any bottom, sandals or flip flops usually.
Round 2 (philanthropy): same as above
Invites: skit people wore their "costume" and everyone else wore matching sisterhood rush tshirts and the same color/style bottoms: black, jeans, khaki, etc.
Prefs: BLACK dress, skirt and blouse, or pants, no flip flops or running shoes (obviously), badge (!)
Bid day: letters, jeans/shorts, and running shoes or flip flops
pearls optional throughout.

Actually, I'm trying hard to remember if anyone wore pearls at anytime other than prefs...