Rusty-James grows color blind because he is depressed at his brother Motorcycle Boy’s death and his proximity to it. Rusty-James makes several references to color and blindness. For example, he says that when he is in a fight he sees red.
During a fight, though, I almost go blind: everything turns red. (p. 23)
This is an example of how when people are angry they can no longer see. When we get overcome with emotion, we go blind. Anger can cause this figuratively, but sometimes grief can cause it literally.
Another example is Rusty-James identifying with Motorcycle Boy.
[His] hearing had been screwed up for years. He was color-blind too. (p. 55)
Motorcycle Boy does not seem to operate within society’s boundaries. An example of this is when he sets the animals in the pet store free, and tries to set the fish free.
When Motorcyle Boy is shot, Rusty-James notices that everything is gray, and he can no longer see colors.
There wasn’t any colors anywhere. Everything was black and white and gray. It was as quiet as a graveyard. (p. 131)
Rusty-James is emotionally blinded and deafened by the incident, so he becomes symbolically so.