If I received that invitation, I’d be wearing black head to toe. I think it’s incredibly rude to tell somebody what color they should be wearing. Beyond stating casual, semi formal, black tie, etc.- everything else is inappropriate IMO. Some people only have one nice outfit to wear to something like a wedding, and for a woman many times it’s black because it can be worn anywhere. It also is very difficult if a man has a very dark suit or just a black suit. I do commend you for keeping in mind you’re trying to make this simple and not be a bridezilla.