Symbolism, Imagery, Allegory
There are so many mentions of light—the way the sun shines, or how the moon glows—that discussing any of these can feel like talking about how much sand there is in a desert.
But ignoring the presence of light and its sources in Ballad would be to skip a step in thinking about how McCullers works. She seems intent on paying careful attention to the outside world, diverting us from noticing how little we know about the emotional lives of many of her characters.
Oh, you want proof? We can give you all the proof you need.
Check out what's happening in the story when these emotional descriptions of sun, moon, and light pop up:
Exhibit A: The red winter sun was setting, and to the west the sky was deep gold and crimson. (Ballad.153)
This rich, colorful description is as heightened as what is about to happen: Miss Amelia is about to discover Marvin Macy is back in town.
Exhibit B: The lamps in the houses made mournful, wavering flickers when watched from the street. (Ballad.50)
These mournful, wavering flickers catch the questionable town gossip, its "chin music": has Miss Amelia in fact murdered the strange little hunchback?
Exhibit C: [...] the faces of people were strange in the fading light. (Ballad.208)
The old Doors classic may say that "people are strange when you're a stranger," but here, the strange faces are transformed by a mix of blood thirst, worry, and curiosity as the sun goes down, their faces more and more shadowed: who will win when Miss Amelia and Marvin Macy fight?
Yup. Basically any time you get a mention of light (or the lack thereof) in McCullers' work, something either emotionally illuminating—or, hey, emotionally dim: McCullers is depressing like that—is happening.