A current trend I love in the wedding world is the mismatched but beautifully coordinated bridesmaids dresses. There are no real rules when it comes to doing mismatched bridesmaids dresses, I think the key is just to have them all along the same theme.
I believe this trend took off after the Sex and the City Movie where Miranda, Samantha and Charlotte all wore different style/coloured Zac Posen dresses to Carrie's (not so successful) wedding. I think before this 2008 movie, people would have looked at you very quizzically if you mentioned you didn't want bridesmaids wearing the exact same style of dress...Nowadays, I just can not understand why maids need to look like cookie cutter clones of each other. We all know women come in different shapes and sizes and not one dress fits everyone, so why not make you maids comfortable and let them wear a style that suits them.
Here are a couple examples of what in my opinion the bridesmaids look cohesive enough.