Residency interviews are a bit different than business school interviews, but my fiancée goes with a plain brown or grey suit (with skirt) and wears a blouse with the colour of whatever school she's interviewing at (Red for OSUMC, Blue for UPMC, Green for Tulane, etc.) I've noticed a lot of the ladies from med school or residency interviews go much more plain with a grey, black, or blue suit (pants or skirt) and a simple creme or white blouse. Typically do inch or two inch heels. A big tip is take a pair of flats if you're not comfortable with heels. When you're away from the faculty and just with students or on the tour, sneak the flats on. Fiancée was dead tired after an eight hour interview day for residency before she learned this trick; but seriously, she should've been able to interview in tennis shoes and scrubs, since that's all they'll see her in for four years.
Going 'interview attire shopping' with her was much more tiring than when I bought my interview attire.
I think I now know more about faux pauxs for women than men