Clothing and fashion are important indicators of social rank and character tn both "Persuasion" and "The French Lieutenant's Woman". Although both novels are set in the 19th Century, the fashions changed dramatically from the day Louisa Musgrove fell off the cob and the morning, fifty years later, when Charles first encounters Sarah staring out to sea.
Notice the difference between Anne's simple muslin dress and the clothes of her sister and father in the following scene from the film adaptation of Persuasion (1:00-2:23).
In this scene from "The French Lieutenant's Woman" the simple, somber dress of Sarah can be easily compared to Ernestina's more fashionable clothes. Towards the beginning of the clip, Mary's serving dress is also visible (2:17-4:40). Does Sarah's dress more closely resemble Ernestina's or Mary's? Is this diffrence a reflection of social status or personal character?
The clothing differences between Sarah and Ernestina are also emphasized in the novel. In chapter 1, Ernestina is described as being "dressed in the height of fashion, for another wind was blowing in 1867: the beginning of a revolt against the crinoline and large bonnet," wearing, "a magenta skirt of an almost daring narrowness-- and shortness... and perched over the netted chignon, one of the impertinent little flat "port pie" hats... a millinery style that the resident ladies of Lyme would not dare to wear for at least another year."
Sarah, on the other hand, wears her "eternal indigo dress", Until Charles meets her years later in London, where she is wearing "the full uniform of the New Woman, flagrantly rejecting all formal contemporary notions of female fashion" (Chapter 46).
Why does this change occur in Sarah? (It may be a good idea research Victorian fashion to fully understand this transformation).
Fashion-Era is a great starting point for researching historical fashion. Check out the sections on the Late Victorian Era as well as the Victorian and Edwardian Seaside to gain insight into the clothes worn in "The French Lieutenant's Woman". Fashion-Era also has a great Regency section.