Help With MOB Dress

I am having my mother walk me down the aisle instead of my father. My wedding colors will be yellow and orange, but I don`t want my mom to wear the same as the rest of the wedding party. My wedding will be very informal, in July of this year. Do you think it is okay if my mom wears a floor length black dress to walk me down the aisle? Please be honest. (Normally the father of bride would wear a black suit so I thought that would look okay???)