Well, naturally, the simple ones like black and white would be colors to use in an essay. Keep in mind that the book deals with shades of black and white too; the government paint, Optic White, has just a little bit of black in it, so there's a shade there. There's also some places where just how black an African American is comes into play, as when you see someone referred to as "ginger-colored" in the novel. You might then describe the importance, whether socially or otherwise, in the book of the color of a person's skin.
If you wish to avoid race altogether, there are a few places I can think of quickly where color comes into play. Among these are the descriptions that are meant to emulate (or allude to) Faulkner, where Ellison describes the college campus in detail. You might be able to use something there.
well i think that the generale idea of this novel(invisible man) that is the narrator was suffering from racism from both white and black people so he became thinking about his invisibility as a result of this racism;as it is a matter of defferences between whites and blacks in society.