My mother in law wants her family to match my future husband's outfit.

Unfortunately, you have no authority over what FMIL or the rest of the family wears. I don't know about Turkish culture but in the US traditional etiquette discourages guests from wearing bright red as being loud and attention seeking. That said, it would only be a reflection on them if they do. Otherwise, there's no issue with any guest wearing what they want to wear whether it happens to coordinate with the wedding colors or not, as long as it suits the formality of the occasion. But, again, you aren't dressing them.

You can really only dictate what the wedding party wears, at least in US and Canada. I would let all of whatever everyone else wears be something you can't control.

I mean, it's his family. Can't FI just tell them that people should wear what they like and shouldn't be pressured to match?

As difficult and as annoying as it is, I would try to let it go or have your fiance address it. I find her behavior utterly rude, but try your best to get over it. Hopefully, other family members will have more etiquette sense. Your future MIL doesn’t even have the sense of a billygoat. Good grief.