Well, this is a great coincidence. She is wearing a yellow dress, with yellow roses, like the Tyrell`s, and she is walking on a green garden (you can say it`s Highgarden), like the Tyrell flag`s background. And she has brown hair, which is the specific hair color of this house. Maybe she is that beautiful cousin about whom Margaery was talking to Sansa. It could be, isn`t she?