The line that stands out the most to me in terms of the imagery it presents through repetition comes after Della has finished crying, and she takes out a rag to dry her cheeks.
She stood by the window and looked out dully at a gray cat walking a gray fence in a gray backyard.
Della is quite, quite down as a result of her meager savings. She really wants to buy her husband, whom she loves very much, a lovely gift for Christmas, and she simply cannot afford it. In this line, she is described as peering out the window dully, and this idea of dullness is echoed by the repeated use of the word gray. Gray is such a dull color, never bright nor cheerful, but always muted. The repetition of the word gray—three times in this sentence, in fact—certainly works to describe the way she sees everything in her life in this moment. Everything, for her, feels gray: dull and muted. She isn't happy, and this sentence conveys that quite clearly.