If a few guys are wearing a tuxedo, you should be more than safe with a dark suit in either navy or grey. White shirt and a blue tie with a grey suit / silver to grey tie with a navy suit. This is a classic for a reason. If you want to wear something with a bit more color, try a pink shirt with a burgundy (burgundy, NOT red) tie.
If you go the navy blazer route, I suggest trying it with light to medium grey trousers. It will look better than a blazer with khakis and is also a better choice for an evening event. A white shirt is best if you are wearing grey trousers, light blue shirt if you are wearing khakis.