It's Your Wedding
The bride always has the final say in the choice of bridesmaid dresses. It's not a dictatorship, though. Bridesmaids are responsible for paying for their gowns, so being considerate of their feelings is important if you want friends after the wedding. After discussing your wedding theme and colors with them, you may want to ask your bridesmaids for general suggestions about their dresses. Although it pays to be diplomatic, your opinion is the one that counts the most.
A full-length bridesmaid dress will cost at least $200, so being mindful of your friends' budgetary concerns is important, too. If you want a friend to participate in your wedding who can't afford to pay for a gown, you can opt to buy it for her. This may be sensitive territory, but if the alternative is for someone special to bow out of your wedding party because of financial concerns, it's an option.