All eyes may be on the bride, but everyone in attendance will be appraising you. You won't have to worry about hairs escaping your updo, and no one's going to be judging the neckline of your dress. But, as the father of the bride or the groom, your attire should set a standard for the wedding's formality.
If the ceremony is black tie, you'll be expected to wear a tuxedo or formal dinner outfit. Black, charcoal and navy are generally considered appropriate colors for formal suits at evening and winter ceremonies. Brown, beige and navy dress are preferred for daytime and summer events; during the day, you can opt for a sport coat, tailored trousers and a necktie. Color options are much more varied at causal ceremonies, but just because you're sporting a blazer doesn't mean you can show up in something you'd wear to the office. This is your kid's wedding day, after all!
Your dress will be determined by the style and location of the ceremony, so even if you've always dreamed of wearing a tux when giving away your daughter, you're going to have to suit up in something more causal if she's having laid-back beach nuptials. You don't have to dress down in flip-flops and khaki shorts, of course, but dress pants and a lightweight sport coat would be more appropriate. Of course, the reverse is true as well, so don't get snappy about tying a bow tie if your son chooses to say "I do" in a formal ceremony at a cathedral.
If the wedding party is adhering to a specific color pattern or theme -- red and white for Valentine's Day nuptials, for instance -- your dress should reflect those colors. In this case, you could opt for a white tux with a red bow tie. You can wear something more traditional if you'd like, such as a standard black tux with a red pocket square, as long as it's not conflicting with the overall look of the wedding party. Along with the groomsmen and the best man, you're setting the style of the event, but you also have to conform to it.
Ultimately, it's the bride who gets the last say about wedding day attire, so seek her approval whether she's your little girl or your future daughter-in-law.