They said it best, match your tie to her dress or if you're wearing a cumberbun match that too or a different color like black or white and matching tie (depending how it will look). Wherever you get your tux or suit should have plenty of colors to choose from unless you just want to buy one and that may be a little hard. I agree with the guy if you get her a corsage have it matching the colors in her dress but with complimenting colors (you should ask her if you wanna do the corsage and boutonniere thing and she'll get you one to match).
Heres what I usually do when I have a date to a formal event; I usually go with black tux or suit depending on the occasion but try to match my shirt, tie, vest to whatever my date is wearing. Also a nice touch is a matching colored flower if its a wedding or in your case prom.
If your renting your suit/tux then tell the guys at the place they might be able to help you out. If you own a suit and want to go the cheap way try to find a shirt/tie combo that match. This is usually the best since you can always wear it again compare to how often you might wear a vest.
You could also go see how much it would be to rent a colored vest to match the date.