As far as being a bridesmaid, your role is to support the bride. That means wearing the dress they choose for you with grace and being by their side during any mini-meltdowns that may occur on the big day.
The fact is that no matter how pretty the dress is, they will always feel like a bridesmaid wearing it. So choose a dress that you like and they feel good in, and hopefully you will both enjoy them on your wedding day.
Different colored bridesmaid dresses are very popular right now. But it can be difficult to select ones that look good together since you're limited to the colors the designer offers. Also, the small swatches on a card often look very different when they are made up in a gown. The easiest way to visualize how they will look together is to check out the full-size samples in daylight, and switch out different options until you are happy with the combo. And consider pre-selecting the color each bridesmaid will wear — can prevent a lot of inter-wedding party conflict.