This hasn't been confirmed, and there are no signs of any Shiny Legendaries over GTS. Note that Zekrom, Reshiram, and Victini have been programmed to not appear shiny. Perhaps Game Freak carried this trait over to X & Y. Besides, Xerneas and Yveltal have cinema that have 3D Rendering unlike the other ones from Gen 1 to 4. Until someone shows proof of this, the answer will be anonymous.