Symbolism, Imagery, Allegory
Crane is a master at depicting grimness. But even though we may associate tenement life with a palette of colors from grey to greyer, Crane uses color to ramp up the drama of the images—particularly red.
For instance, Momma Johnson is red a lot. That's because she's an angry drunk. Our first image of her includes the description that her "neck flared suddenly crimson" (2.26), and moments later we know she's not only drinking, but ticked off because "[t]he fervent red of her face turned almost to purple" (2.48). Their surroundings might be drab, but Mom's anger is about as dramatic and vibrant an emotion as exists, it seems.
Momma always appears crimson, irritated, or otherwise reddened. So, by process of elimination, we know that red doesn't represent love and roses—it's more like violence and wrath in this book.
The violent connotations of the color red exist from the first. When we meet Jimmie in the beginning of the book, he's all beaten up with "[b]lood […] bubbling over his chin and down upon his ragged shirt" (1.6). And this pretty much never changes. From the beginning to the end of the novel, Jimmie is either bleeding, watching someone bleed, causing someone to bleed, or all three. This near-constant presence of blood—and its accompanying red color—ensures that we understand Jimmie is angry and violent, and also that the color red is associated with these things.
But let's dig a little deeper, shall we? At one point in the book, we're told about a woman performing in a dive bar: "A ballad singer, in a dress of flaming scarlet" (12.1). Maggie is in the audience watching with Pete, so her fall into total despair hasn't yet happened. But this singer's dress is a bit of foreshadowing for what's to come for our main girl, and the color lets us know it isn't good.
There are two words that do all the foreshadowing work when it comes to the singer: "flaming scarlet." Flaming indicates brightness, yes, but it also refers to destruction—fire destroys. So what we have here, then, is brightness—visibility, vividness, that sort of thing—directly connected to total destruction. In daring to engage with Pete the way she does, Maggie is daring to dream of a better (or brighter) future for herself, one in which she lives life more fully. But at the same time, she is shirking social expectations and, because of this, ruining her reputation.
But if "flaming" does all that, then who needs "scarlet"? Here's the thing about scarlet: As much as flaming refers to the connection between brightness and destruction, scarlet genders this reality. Ever heard of The Scarlet Letter? In short, as a color, scarlet indicates that a woman has misbehaved—especially sexually. Which, of course, is exactly what everyone thinks about Maggie at this point and going forward. The singer's outfit, then, reflects not only her own questionable morality, but Maggie's as well, and subtly clues the reader into the fact that things can't possible end well for our main girl.
Now go eat a strawberry or something—there's plenty of awesomeness to be associated with red in the world, just not in this book.