In the past it was considered a must that all the bridesmaids' dresses match. And this might work well if all of the women in your wedding party are of a similar body type.
But more often than not, this isn't the case, and having identical bridesmaid dresses inevitably ends in one of said bridesmaids looking uncomfortable and odd.
Thankfully, it's become much more common in the last ten years or so for brides to choose a single color and have all the bridesmaids wear a different style of dress in that color
Then, there are the brides that dispense with matching all together and have each bridesmaid in a different style and color that suits her best, according to a unifying theme.
Speaking of a rainbow array...great idea. Look at the matching flowers for each bridesmaid. Notice how even the shoes are all variations on a theme and help pull the look together.
This bride chose to have her bridesmaids in a vintage style, but each bridesmaid is wearing a slightly different length and cut. The dresses are tailored for their different heights and body types, and small details are altered in each. Oh yeah, and they're all wearing completely different prints and colors. The accessories and flowers, again, help unify the look. I love this one a lot.