My loyalty lies with your closet friends in this case. Their concerns are very rational and legitimate, and I think it's important to ensure that they feel comfortable in whatever it is you require them to wear.
I understand that you're concerned about hurting your mom's feelings, but the last thing you need during the wedding planning process is this kind of stress. I would kindly sit your mom down and say, "Mom, thank you so much for being involved in this process with me. It means so much to have your support and to factor your opinions into my decisions. With the dresses, I've decided that my bridesmaids should wear something else. They are worried about wearing white, and they would be more comfortable in something else that is more their style. I want to find something that all of us can agree on. I'm sorry you spent so much time looking for those dresses. Let me know if you want help reselling them on craigslist, ebay, or Etsy or if you want to save them for future occasions."
In situations like this, it's important to follow your heart (in a way that respects other people's feelings, of course). Your mom's feathers may be temporarily "ruffled" but none of it will matter when she sees you standing next to your beloved, professing your love and commitment!
Best of luck to you...
(and please, 2000dollar kindred spirits, chime in with your opinions!)