Keira Knightley wears several striking outfits in the new period drama “Colette,” but the director Wash Westmoreland’s favorite fashion moment in the film has nothing to do with the character’s jaunty straw hats or menswear-inspired ensembles.

Instead, Mr. Westmoreland said the key to “Colette” comes just as Ms. Knightley’s turn-of-the-century title character prepares to meet the men and women of Parisian high society, then idly notices a toothpaste smudge staining the bust of her dress.

“It’s a comment on the philosophy of the movie,” Mr. Westmoreland said in a phone interview. “Yes, it’s a costume drama, but the costumes have stains on them because the people have real bodies. And real bodies make a mess of things.”

Though she was a fashion icon in her day, Colette surely would have appreciated that subversive spirit. She is now best known for writing “Gigi” and “Cheri,” but Mr. Westmoreland’s film tracks Colette (1873-1954) long before she became a famous author in her own right, when she was just a young girl plucked from the Burgundy countryside by her showboating new husband Henry Gauthier-Villars (played by Dominic West) and plopped into Paris. There, Colette labored in secret to produce novels under her husband’s nom de plume, “Willy,” but chafed at his controlling nature and expectations that she should be a pliant society wife.