The question is: what makes your husband so uncomfortable about this person's clothes? Unless there was reason, I would most certainly *not* ask her about any gang-affiliation, and by "reason" I mean questionable things happening, weird friends dropping by...in short, anything that might be an issue with ANY nanny, regardless of dress.
If it were me, I'd suggest to my husband that he take a half-day at home, just to hang out in the background and listen to her interactions with the kids. If some specific complaint were to come out of that, well, then, time for a conversation.
I know that, as a young nanny and preschool teacher, I often wore unusual clothing combinations. I don't spend a lot of money for my nanny or preschool clothes because chances are, the kids are going to get something (paint, snot, etc.) on them, so wait and see how she's working out before making an issue out of her apparel.