The Empress Eugénie had a great attachment to the aesthetics and royal artifacts of the eighteenth century French court and, especially, a fascination for Marie Antoinette. She would frequently appear at costume balls wearing gowns based on or inspired by those to be seen in portraits of her tragic royal predecessor. This theatrical image of Eugénie was painted only a year after she became empress, and is one of the lesser known of Winterhalter's paintings of her.
Below is a photograph of the Empress wearing the same costume as she wears in the painting. Since this obviously isn't a portrait photograph, it's likely that it was taken for Winterhalter to use as a reference for details of the costume.