There are a couple allusions to painting throughout the novel. The big one, of course, is when Bobby paints the mural on the wall (and then gets arrested). Basically, what he paints is a giant self-portrait: a ghost boy with a faceless baby in a carrier. We're told:
Pretty soon he's going to have to look inside the carrier and make up a face for the kid if it's gonna be following him all over the damned place anyway.
He's going to have to see it. (15.23-24)
We can look at this faceless baby in the mural a couple ways. It could be that the faceless baby in the carrier symbolizes Feather's lack of identity so far—she's new, and Bobby's still figuring her out. The facelessness can also be seen as part of Bobby's own quest for identity, his own unresolved understanding of himself as a father. He, himself, is metaphorically faceless as a parent at this point. Lastly, the facelessness can be understood as resistance: Bobby's not putting a face to the rest of his life; he isn't looking it in the eye.
Now let's look at Bobby's materials for a moment, and how they interact with the faceless baby:
It's all got to come to an end soon. I'm going to have to find the kid's face. It's going to be hard now 'cause I'm out of breath and running out of color in the cans.
I'm almost empty.
But I got to find the baby's face. (15.29-31)
Bobby's cans are almost empty, and while this is literally true, he's also speaking figuratively here—Bobby himself is almost empty, depleted of his childhood and running on fumes are he tries to figure out how to be a dad. His life has changed so much that he doesn't know who he is anymore, so it makes sense that he sees himself as an empty person, someone who's lost their color. That this coincides with his search for the baby's face makes it clear that Bobby's feelings of being lost are intimately tied to his struggle with becoming a parent.
The mural isn't the only reference to painting that Bobby makes, though. When he sees the posters of the perfectly happy family in the social worker's office, he thinks:
I want to spray black, greens, and reds all over this office and cover the smiling faces of the kids and the grown-ups.
What the hell are they smiling for? (22.16-17)
In this case, Bobby literally wants to cover up what he thinks isn't representative of reality, to pain over what he recognizes as phoniness. Bobby doesn't particularly want a baby in the first place, he's not sure if they should give the baby up for adoption, and he doesn't even really want to make a decision about the baby. The smiling people are just a little too fake for him—this isn't a happy place from where he's sitting. In wanting to cover the images up with paint, then, Bobby wants to reveal the truth. His truth. Which is, of course, what he's also working out with the mural.