One of Maniac Magee's major faults is his inability to recognize and understand racial differences.
He looks at his black friends' skin colors and notices that they come in a vast array of shades, which he describes poetically as "gingersnap and light fudge and dark fudge and acorn and butter rum and cinnamon and burnt orange." And he looks at his own skin and doesn't see it as white, like a sheet of paper, but as many different shades. He even thinks about how his eyeballs are just as white as those of his friends, and doesn't realize that this similarity means nothing to the racially segregated East End and West End areas of his town.
You might argue that Maniac's color blindness is sort of a good thing: he treats people equally; he judges people based on their behavior rather than the color of their skin. That's admirable. And yes, Maniac is absolutely correct to notice that skin tones run the gamut and aren't just "black" or "white." But what he doesn't recognize is that his pale skin makes people perceive him as different from the African Americans who live in the East End, in Amanda's neighborhood. This ignorance of racial realities makes him unaware of why his presence there isn't tolerated by everyone, such as Mars Bar.
In Chapter 16, as the narrator explains, "Maniac kept trying, but he still couldn't see it, this color business." Although he's a good reader, and a helpful tutor for Lester and Hester, Maniac is simply ignorant when it comes to this aspect of reality. It's why he seemed to react so stupidly to the old man's insistence, in Chapter 17, that Maniac go on home to "his own kind." He doesn't know what "his own kind" means.