Look and skin color never changed... However, accent do changed.
I too have a BIL (a brother of my BIL) who married a White girl. If she was to speak on the phone with you, you would swear she's Hmong - with the Hmonglish accent too. Once I was talking to her and I told her that she speaks Hmong better than English - she laughed at the comment and said I'm not the first one to tell her that.