My daughter changed into black leggings and a black top for a couple (movement call/call back) - but I guess maybe she was supposed to for those schools. She wore a plain black cotton dress otherwise -very plain and simple. Didn’t seem to make a difference (she had good results and several acceptances, but did have rejections too — doubt any of them were related to her outfit).
Several programs have their students wear blacks to theater class (which usually means leggings and a t-shirt). I would think most auditors are used to seeing actors in black leggings and a top…
Wear what you are comfortable in. Just make sure the leggings and top look nice and neat and pair with nice flats or something. You may not see others in leggings, but that is ok. We saw just about any type of dress you can think of (jeans, cocktail dresses, nice pants, short skirts, long skirts, casual dresses, etc).
You have to wear what works for your monologues and the movement may be the key that gets you noticed by a program. 
I’m sure they would rather you be in leggings and be able to truly move than be restricted in other clothes. Especially some of the programs that really focus on movement (that is what they are looking for!). good luck