Mismatched Group Groomsmen Shoes and Belts

I personally prefer cohesion, so I'd prefer all brown or all black belts and shoes, but I think asking for the belt and shoes to match is a nice compromise!

I'm going out on a limb here. . .tread lightly. . .it may come off as micro-managing. You've already asked them to purchase a suit rather than rent one. I would have assumed your belt/shoe color selections would have been presented to them when you provided the info about the suit. Some budget differently and some may not have multiple pairs of dress shoes. Being so close to your wedding I would ask the GM's to have their belt and shoes match and suggest it be your preferred color choice, but requiring it could cause a problem.

Did you pick a rental suit or are you having them buy suits? With rentals you should also be able to add matching shoes/belts to the rental. We considered saying wear your own brown shoes and belts to save them money because I personally don't think it will matter if they all have the same exact shoe as long as its the same color family. We ended up adding the shoes to the rental though because my husband tried them on and actually really liked them and wants to wear them himself (he HATES rental shoes normally) and thought that was actually easier for his friends anyways. We also decided against including belts in the rentals and instead we're buying them suspenders.

If you're concerned about cohesion and it will work with their suits, I would ask them all to wear black instead of brown. More people own black shoes than brown (my husband had to buy brown shoes on the way to an out of state wedding he was in right before covid) and there aren't as many shades of black as there are of brown, so you have a better chance of everyone wearing something similar if you ask them to wear black.