The colors of her military-inspired dress are American rebel uniform colors. Her husband Bonabes was killed on the vesselZéléwhile returning to America as part of the French forces there in 1783. They married in 1777 and I do not know when he was first posted to America.
From batguano.
Nathalie is dressed showing support for her husband's military engagement in Britain's colonies. She may be wearing a fur hat, also reflecting military influence. See a comparable portrait of Lady Worsley here.
Keywords: 1787, Vigée-Lebrun, Marquise de Rougé, de Mortemart family, de Rougé family, Marquise, French, long curly coiffure, feathered hat, chemisette, vee neckline, collar, neck tie, jacket, lapels, long tight sleeves, buttons, vest