Whenever I see portraits back in the 18th century, they always have white hair. I don't know why they have white hair. Probably y'all be like, well its obvious, the person in the portrait is old. No! They are not old! They are young! Why do people have white hair in old 18th century portraits?
Those are powdered wigs, or hair powdered to look like wigs.
/u/mimicofmodes wrote a great post about the rise of wigs (among other things) in fashion here.