A woman with a red scarf, white shirt, and tan shorts is giving a shushing sign to the camera in front of shelves of books.
a woman in a scarf
"A woman in a scarf" is a less descriptive way of describing "a woman with a red scarf, white shirt, and tan shorts."
A person who is with a scarf is in the scarf.
A woman in a scarf is a rephrasing of woman with a red scarf.