We're allowed to wear matching shirts, like our letter shirts. We always order enough theme shirts for everyone (they come out of dues). We're not allowed to wear anything else that matches, though. Inevtiably, two or three girls are going to be wearing the same black cocktail dress for Pref, but if it's everyone, we could be in big trouble with Panhelleni
We're also not allowed to wear anything that could be constituted as a costume -- so, like, no matching hats, no completely matching outfits, nobody dressed up as the mascot, whatever. This year we did a country club theme and we all wore fake pearls, and there were some people who thought Panhell might come down on us for it. Fortunately, they didn't (and they didn't all match, so we could've probably gotten away from it).
Anyway, the chapter is really opposed to having to pay out of pocket for rush clothes, so we would never be able to say "okay, everyone go buy j. crew flip-flops in xyz style". We asked everyone to wear white bottoms for the same country club theme, and a lot of people got really angry because they didn't already own them. (I spent a lot of time explaining the "wear it with the tags on" concept.
) So we try and keep it to jeans or black pants as much as possible.