Emma is wearing a red halter Giambattista Valli gown with bow detail on the halter. Nicole Kidman wore this exact gown too. In my opinion both look amazing in it. I love the red color and the bow on the halter.
Emma Stone Photos - Emma Stone attends the "Charles James: Beyond Fashion" Costume Institute Gala at the Metropolitan Museum of Art on May 2014 in New York City. - Red Carpet Arrivals at the Met Gala — Part 3
Emma Stone's 19 red carpet risks that seriously paid off