If it’s not to late, work out a compromise. Sure it’s your wedding, but it’s their self-esteem (and probably their cash, too), and you need these ladies to love you long after you become Mrs. Married. Tell them you’ll pick the color and then let them choose the style. If a bridesmaid is being unreasonable (make sure it’s not you being too demanding), let her know how important your vision is and that you will work together to find a dress that will make you both happy.
You hear grumbles about expenses.
Be considerate. It’s likely that your maids will only wear this dress for a few hours, so don’t make them hock their car to be a part of your wedding. Brides are not required to pay for the dresses, but if you want to spring for something pricey, consider adding it on to your own budget or paying for half. Try to mitigate expenses elsewhere too -- if they’re buying the wedding bridesmaid dresses, don’t make them buy jewelry and shoes too.
It’s never easy, but sometimes when all else fails or when something explosive has happened, the only alternative is to ask a bummer bridesmaid to step down. There really is no right way to do this. Good friends will understand the decisions you have to make. People who’ve turned out not to be such good friends are better weeded out, anyway.