It's the bride's privilege to ask her wedding party to dress a certain way for her wedding. It's tradition. She can tell them what jewelry and shoes to wear, too. She can even ask everybody to wear their hair up, or down, if she's super picky. But she absolutely, positively CANNOT ask anybody in her wedding party to alter their physical characteristics in any way.
Being asked to be a bridesmaid or a groomsman is one of the highest honors given to a friend. Yes, it is a lot of work and sometimes it isn't fun but when your friend asks you to stand with them while they take this huge step in their lives, you do it... and you do it without any complaints. Unfortunately, that isn't always the way things go.