Symbolism, Imagery, Allegory
To contrast with the "inner richness" of Della and Jim's love for each other, O. Henry throws in lots of little details to make their external circumstances about as drab and meager as can be.
There's the flat itself, with its malfunctioning mailbox, dead doorbell, worn red carpet, and cheap mirror equivalent. There's the dull scene out the window: a "gray cat walking a gray fence in a gray backyard" (6). There's Della's "old brown jacket" and "old brown hat" (11), and Jim, with his worn overcoat and gloveless hands. All of this imagery creates a contrast between the rich, warm, inner world of love and affection which Della and Jim create, and the gray, ugly, outer world of money and work and miserly, hair-buying business owners. Their love transforms their flat from a particularly drab part of that dreary world into a home.