It is often the case that the clothes we wear say a lot about us. They are one of the many different ways we can express ourselves and reveal our personalities. They can also tell us about what kind of lives people lead, what social backgrounds they come from, and so forth.
That certainly seems to be the case in relation to the narrator in "Biography of a Dress." She's looking at a black-and-white photo of herself when she was two. In that photo, she's wearing a yellow dress, and it soon becomes clear why the narrator feels the need to provide us with this information.
Yellow is the color of boiled cornmeal, which is what the narrator's mother was always eager for her to eat. Straight away, this tells us about what kind of background the narrator had; it's evident that she was brought up in a poor environment. Our initial impressions are confirmed when the narrator, now a forty-three-year-old woman, tells us that her mother bought cornmeal because it was cheap.
In her mind's eye, the narrator links the color of the dress she wore as a child with that of the cornmeal she used to eat every day at that same time. As with so many other people, her clothes said a lot about her and her life situation.