Never choose dark flowers for your bridesmaids if they are wearing dark colored dresses or light colored flowers for your bridesmaids if they are wearing light colored dresses. You do not want your pretty flowers to blend away.
Keep this rule in mind when choosing your bridal bouquet too. If you are wearing a shade of white make sure your bouquet has some color (this can be achieved with greenery) so it does not appear to be a white blob in your wedding photos.
Image by True Love Photo