When we made weddings, we never told the family men what color to wear.
BUT, in the photos, it does look better if all the suits are the same color. If you are a Yeshivish family, well then, they are all in black, if not, the men wear what they own.
If they are all buying new suits, and they ask you, then you can give your opinion, BUT, never ask people to spend their own money to please the families of the Simcha. Now, if you offer to treat everybody, then that is a different story.