There is no protocol when it comes to a mother-of-the-bride dress. You must wear what you love and what looks good on you. Generally the bride will dictate what her mother will wear...but today, anything goes.
HERE ARE SOME POINTERS FOR MOM'S DRESS:
Whichever style you choose...make sure you feel great in it. It's going to be a long day and/or night. You want to look great and feel great.