I was the same way, I only have a MOH, and I told her that it didn't matter to me what she wore. Whatever she wanted was fine with me. She kept pressing me, so I finally suggested maybe a color to blend with my colored shoes and necklace. I also knew it was a color family that would look amazing with her skin tone and hair color, so we shopped online together for a dress that she liked and found a fantastic convertible dress online at Target for $29.
As a MOH or bridesmaid, I can see where a little guidance in the way of a color or at least color family would be helpful.