Bridesmaids/moh Partners

It really doesn't matter what colour he wears.

Are the groomsmen wearing matching suits or are the ties the only thing they have in common? If the ties are the only thing to tell them apart from other male guests and it's important to you that they look different, then maybe suggest the MOH partner wear a tie in a different colour that will still coordinate with her outfit. But if the groomsmen are wearing matching suits etc, then even if MOH partner has the same colour tie, he is still not going to look like a groomsman. Also, unless he buys the exact same tie (same brand and colourway) then he's still not going to look exactly the same.

He doesn’t need to match the wedding party, and it’s usually better if he doesn’t — otherwise it can look like he’s part of the group. He can choose a tie that complements the colours without being identical, like a softer shade or a neutral that coordinates. That way he still looks cohesive with his partner but doesn’t blend in with the official party.