White/cream/light beige/pale taupe are all apporpriate for mothers of. The usual suit dress styling plus hat is not the least bit likely to be seen as mimicking a bridal gown and veil! If the bride is in full on wedding costume, and the mothers of are also in suitably 'mother of' suits or dresses, they arent competing anyway.
The nobody in white but the bride rule matters more when everyone, including the wedding party is wearing Sunday best type clothes, or super casual clothes, or evening clothes. Being in white will mark her out as the bride if she is dressed in the same sort of non-costumy clothes as the guests. Even then, the rule is more for guests who, if they are in her own age range, might be mistaken for her, or be assumed to be trying to make some bitchy point concerning themselves and the groom!
My daughter got married outdoors, and because of the time of year/angle of the sun/largest shady patch for guests to sit in, she was going to have to walk down one side of the seated guests to get to the back of them to come up the aisle. She asked a bunch of her girlfriends to wear white. Her flower girl and adult bridesmaids left the brides dressing room (also known as the park office) in a casual group to walk round. Then, surrounded by about 8 women in white outfits, including one in her mother's wedding suit from 1957(!), she walked round with a feeling of not being on view yet. She is only 5'3", and they were almost all taller. It worked quite well, and the photographer caught quite a nice shot of them all as they came out. The women went up to sit in a row resrved for them, then us parents and the usual procession.