The wedding couple will, in most cases, have clear ideas about what they want.
Give your groomsmen very precise advice, suggestions, and demands. “I want you to wear this, and I don’t want you to wear that,” tell them. They’ll know exactly what to do after that.
Consider how closely you want them to resemble each other. Is it required that they all wear the same tuxedo? Or are you okay with them wearing the same bow tie, pocket square, and vest? Getting the same accessories for continuity is a good idea.
Make sure the groom stands out from the rest of the groomsmen. If the groom wears the same tuxedo as his groomsmen, the groom will easily blend in. One way to do this is to wear a different coloured pocket square or a different coloured boutonniere.
A vest is a good item because it typically has deeper cut armholes, so you don’t have to think about sleeve length. All you have to worry about is chest width, which is easy to get correct.
Colored shirts should not be worn, particularly by the groom. They can be red, green, blue, turquoise, violet, or purple, depending on the wedding theme colour. That, on the other hand, is a little excessive. If you want to add any colour, use a pocket square or boutonniere flowers; otherwise, a white dress shirt is formal and appropriate for the occasion.
As a groomsman, you stand out among the other guests, but you never want to make the groom or the bride look less dapper or less well dressed than you. Try not to overdo it with your tie knots or something else that could make them feel awkward or like they aren’t the centre of attention because it’s their day. Maintain a straightforward approach.
Also, avoid wearing any unusual shoes that would attract too much attention. Your best bet is probably a pair of black cap toe oxfords. Otherwise, make sure you get over the calf socks that stay up all day, even though you walk a lot, because no one wants to see your sweaty calves at a wedding, which is normally very formal.