La Duchesse de Polignac, Yolande Martine Gabrielle de Polastron. She was the favorite (most favored person within court, essentially a best friend) of Queen-consort Marie Antoinette of France from 1775-1785, and again in 1789 just before the start of the French revolution. Yolande was a terrible influence on Antoniette, encouraging her into affairs, vices and extravagant over-indulgence. This as well as Yolande's political association with the ultra-monarchist movement led her to be despised by revolutionaries, which drove her to exile in Switzerland after the storming of the Bastille. Revolutionary propaganda even baselessly suggested that Yolande and Antoinette were lesbian lovers!
For the pleasure of Keira Knightley fans, Yolande was actually also very good friends with Georgiana Cavendish Spencer, the Duchess of Devonshire.

Art by Louise Elisabeth Vigee Le Brun.
Her hair is not in a wig, it is powdered, as was common to wear in less formal portraits. This is why it looks less white and more ruddy grey with the undertones of her dark brown hair color coming through. Her eyes were a rare lilac/violet color.