Black or Navy Suits?

I thought about it, but I just thought it might be too much gray..

I would say they should wear black or gray suits. I would refrain from using any color that's not part of your color scheme to begin with. Maybe that's just me but it would seem out of place.

I would do black since navy isn't one of your wedding colors. Black is also more formal than navy.

If its formal, for sure go black!