Symbolism, Imagery, Allegory
No shades of gray here—in James's mind, everything is either black or white. He never said he wasn't an extreme dude.
In this book, white often represents something that is clean and healthy. The first room James is led to, for example, is so white and bright that he's inspired to declare: "Everything is white" (1.2.48). The dentist's office is even "cleaner and whiter" (1.7.209).
Sounds good, right? Of course, part of the rehab process involves suffering through pain in order to get better, which may be why white also symbolizes pain, for example when James sees the dentist: "There is white. Everywhere there is white. There is agony" (1.7.319). In this book, it's no pain, no gain.
Now, when James gets extra crotchety, he starts talking about black as though he's a high-school goth kid. "In the terror of my Addicted mind, all is Black" (2.4.229), he says, trying to convince us that he's comfortable with this tortured addict persona he has created. He's proud of it, too: "I like Black goddamnit, and I am going to give it its due" (2.4.229). This is said without any trace of irony or sarcasm.