If you're lucky, the bride will ask you for input on choosing a bridesmaid dress. Feel free to comment on things like color, pattern, fabric, price or hemline length. Instead of saying you adore or despise a dress, without constructively explaining why, try framing your feedback in a way that's based on objective factors: dress availability, affordability, how flattering it is to your figures, or whether the color compliments everyone's skin tone (marigold yellow doesn't look good on everyone)
Ask her what she thinks before sharing your thoughts. Don't slaughter a dress under consideration; instead, keep your assessment "soft," because she might select it.
But make no mistake -- she is under no obligation to ask for your point of view. Don't expect to be asked. Accept her decision happily. If you love the dress, rave to the bride about her superb fashion acumen. If you're not so lucky, suck it up and be thankful you have to wear it only one day.