Do my ushers have to dress the same as my groomsmen?

[Aquababy80]

I'm getting married in August in a morning wedding and we are keeping things fairly informal. My bridesmaids are wearing black knee-length dresses, and the groom and his groomsmen are wearing basic black suits with white shirts and matching black/white ties (the wedding colors are black and white, btw). We are going to ask the groom's two brother-in-laws to be the ushers. I'm not sure if they already own black suits, and I don't want to make them buy or rent one if they don't. Is it acceptable to have ushers who wear whatever suit they have available, or should I try to have them wear black and should I buy them ties that match the groomsmens as well? Help!