The big question: suit or tuxedo? Well, there’s no rule so it’s up to you.
If you opt for a tuxedo, as a lot of people do at the moment, blue is worth considering. You can also swap the bowtie for a tie, or wear shoes that aren’t patented. Pocket square? Also your call.
If you would rather wear a suit, go for a navy or a charcoal cut from a top quality wool. One or two-button. Paired with an immaculate white shirt of course.
If there was a rule it would be not to blow it all in one go. Freshen up your outfit throughout the day by switching your leather shoes for patent ones, or your tie for a bowtie. You get the jist.
Careful not to dress too young: a suit cut too slim, a tie on the skinny side, overly tapered trousers. You’re not the groom after all. So if he’s wearing a navy suit go for a grey one. If it’s grey go for the navy. Now that we’ve dealt with what not to do, let's get to the fun stuff. Those prized cufflinks for example, it’s time to get them out. None will hold it against you.
You have a free hand. A bright blazer, a polka dot shirt, even an immaculate pair of white canvas shoes - they are all yours. Don’t go completely rogue though.