Which sentence early in "The Gift of the Magi" uses imagery and repetition to reflect Della's mood?

A sentence that includes imagery and repetition is the following:

On went her old brown jacket; on went her old brown hat.

This sentence repeats the words "old brown" and includes the imagery of the old, drab-colored clothing that Della must wear because she and her husband are so poor. The repetition of the words "old brown" emphasizes how depressive and worn-out her clothing and her apartment are. The "old brown" of Della's hair is in contrast with the "brown cascade" of her lovely hair. While her hair is also brown, like her clothes, her hair is glorious, almost to the point of being a luxury. Therefore, her decision to cut off and sell her hair is the greatest sacrifice she can make, as her long, brown hair is her only possession of any value. The "old brown" jacket and hat that she has to wear highlight the brilliance of her hair. 

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.

The story begins with what becomes a melancholy refrain. Della would like to buy her husband a nice Christmas present, but she only has one dollar and eighty-seven cents.

ONE DOLLAR AND eighty-seven cents. That was all. And sixty cents of it was in pennies.

The narrator repeats the amount of Della's meager savings three more times.

One dollar and eighty-seven cents. And the next day would be Christmas.
Tomorrow would be Christmas Day, and she had only $1.87 with which to buy Jim a present. 
Only $1.87 to buy a present for Jim. 

The reader can not only visualize the sum of $1.87 in small coins but can also picture Della counting them over and over again, as if she were hoping as if by magic they would amount to a little bit more. This would seem to be a very good way to represent the whole setting and reflect Della's mood. O. Henry must have thought the same, because he repeats more or less the same words four times. The reader understands not only Della's sorrow but her strong motivation to get Jim a really good Christmas present in order to show her love and to give him some enjoyment in life after a long year of working for a miserable twenty dollars a week. Della keeps counting her money because of her motivation, and her motivation will make it credible that she should reach the sudden drastic decision to sell her beautiful long hair.

Why does the setting reflect Della's mood early in "The Gift of the Magi?"

Della is very sad because

Tomorrow would be Christmas Day, and she had only $1.87 with which to buy Jim a present. She had been saving every penny she could for months, with this result.

She wants to purchase a gift for her husband, Jim:

Something fine and rare and sterling -- something just a little bit near to being worthy of the honor of being owned by Jim.

Della adores her husband, and she longs to buy him a present that is beautiful, something of which he can be proud, and she knows that one dollar and eighty seven cents is nowhere near enough to purchase such a gift. And yet, this was all that she could save despite trying for several months. It is for this reason that Della decides to sell her long and lovely hair to a wig maker; this gets her enough money to go and look for something wonderful for Jim. Ultimately, she buys him a gold watch chain to go with his magnificent watch, but— ironically—Jim has sold his watch in order to buy her some beautiful tortoiseshell combs for her hair.

As the story opens, Della is feeling frustrated and despondent about her state of poverty. We are told she would like to "howl" and "sob" over her lack of money. Repetition reinforces this. The narrative repeats several times how little money Della has to spend: $1.87. She knows that she has saved down to the last penny, and it is still not enough to buy her husband a gift worthy of him.

The setting reflects the financially tight circumstances Della lives in. We learn her flat costs only $8 a month (obviously this reflects pricing of more than a century ago, but it is still low even for that time). We learn that her sofa is "shabby" and that her husband's income has been been reduced by a third, from $30 to $20 a month. Della's mood is bad because, with the finances suffering, she cannot afford a decent Christmas present for her husband even though she has been saving every penny for months. She looks at her surroundings and sees shabbiness and want.