I would say heed the advice of those on campus and maybe look to see if her school has a website/facebook fan page with a 'what to wear' section. That could give you a hint. The positives of black are that it is a neutral, it is easy to find, and it is not any sorority's "color", so it can't be misconstrued that your daughter wants that chapter based on the color dress she is wearing.
Though my .02 (completely anecdotal, of course) is that for my campus and chapter, I have no idea what color dresses the women I preffed were. First of all, I was super jittery with nerves because it was my first time recruiting people, second, I was trying to remember as much as I could about each girl so as to not repeat topics and have a more in depth conversation, and third, it is dim in our room for pref night for the ceremony, so it would have been a hard call anyway. So basically, the dress, at that point (and for me, as I can only speak for myself) did not matter. I'd imagine all the women I preffed were indeed wearing one, but I personally had a lot else on my mind that night.