How we cite our quotes:
Her glance moved beyond the white man's face to the passengers seated behind him. Four or five black faces were watching, two belonging to soldiers still in their shit-colored uniforms and peaked caps. She saw their closed faces, their locked eyes, and turned for compassion to the gray eyes of the conductor. (1920.19)
Helene's encounter on the train suggests that race doesn't instantly create a sense of community or shared experience. The black soldiers are unwilling or unable to help her out of the situation with the conductor. (Mind you, they may need to be careful, too.) This reminds us to resist the urge to lump all people of one race together without accounting for differences and distinctions.
When he first came to Medallion, the people called him Pretty Johnnie, but Eva looked at his milky skin and cornsilk hair and out of a mixture of fun and meanness called him Tar Baby. (1921.32)
Tar Baby's "whiteness" comes up at several points in the novel. For the people in the Bottom, he is the minority, not them. Note the irony of Eva's naming him "Tar Baby."
Nel was the color of wet sandpaper – just dark enough to escape the blows of the pitch-black truebloods and the contempt of old women who worried about such things as bad blood mixtures and knew that the origins of a mule and a mulatto were one and the same. (1922.9)
People who suffer from racism and discrimination are also capable of racism and discrimination. The black residents of the Bottom condemn interracial relationships and liken the offspring of such relationships to mules. This passage suggests that, for some, there is such a thing as not being black enough.