White is for the wedding party and the bride of course, not anyone else. Sometimes it's a major faux pas, and sometimes people say they don't care. Do yourself a favor and avoid any kind of issues with white and select any other color, even black! Black is very much acceptable, just as long as it doesn't look like funeral attire with a veil or hat!
Follow the rule of making sure your dress comes no less than a hand length above your knee. And when it comes to cleavage, be very reasonable. Make sure you are NOT falling out of your dress and make sure that you do not have a plunging neckline. They do make dresses that are classy still, just shop at the right places.
Make sure that you wear something that you can stand in and more importantly, dance in! This means no killer stilettos. Sure, they might look good but you will be miserable within a few minutes. Choose some cute flats if they go with your dress, or some reasonable wedge shoes.