"Gentlemen of the jury, [. . .]. Look at the shape of this skull, this face as flat as the palm of my hand—look deeply into those eyes. Do you see a modicum of intelligence?" (1.13)
You may be surprised to learn that this guy is supposedly trying to defend Jefferson. To do so, he's calling on deep-seated prejudices about race and intelligence. Supposedly, his African features will convey to the jury that he has no human intelligence and therefore cannot be held responsible for his supposed crimes. Yuck.
"Don't be a damned fool. I am superior to you. I am superior to any man blacker than me." (8.39)
This might be a case of someone trying to convince himself of something. Professor Antoine protests too much about his so-called superiority: he judges people in, ironically, the exact same way he's being judged. It's as though he wants to believe that he can order the world based on degrees of blackness, but deep down he knows that superiority has a lot more to do with character than surface.
He hated himself for the mixture of his blood and the cowardice of his being, and he hated us for daily reminding him of it. (8.34)
Grant remembers his own teacher, Matthew Antoine, who was mixed race. This shows us that racial prejudices and stereotypes don't have to come from outside—Professor Antoine has internalized society's prejudice and holds it against himself.
Vivian had met and married a dark-skinned boy while attending Xavier University in New Orleans. She had not told her people about the wedding, because she knew that they would be opposed to it. [. . .] Her family had nothing to say to her husband and hardly anything to say to her. (15.30)
Vivian has light skin, and even though she is in love with a darker dude, she anticipates her own family's prejudice and doesn't make an effort to overcome it. In a way, her hiding her marriage from her family made it that much easier for them to pre-judge her husband, and probably made their lives together a real drag.
Of the three of them at the jail, I figured he was the most likely to be honest with me. He was nearer my age, and he seemed better educated than the chief deputy or the sheriff. And I had heard from people in the quarter who knew his people that he had come from pretty good stock. (17.2)
And so that we can see that prejudice can go both ways: Grant sizes up his three options at the court house and, based on age, perceived intelligence, and family reputation, chooses whom he will trust. Luckily he chooses correctly; Paul is a stand-up guy. However, we can see here the way that prejudices help everyone in Bayonne navigate social relations.
"You go'n buy that?"
I looked around at the short, stout, powdered-faced white woman.
Her face changed, but only a little. (22.89-92)
Whoa, racial profiling. This shopkeeper is doubting Grant's purchasing power because he's a black man. This is a double dose of prejudice: the prejudice against black people and the prejudice against poor people. It's gross, and it's unfortunately still happening today.
"The last thing they ever want is to see a black man stand, and think, and show that common humanity that is in us all. It would destroy their myth." (24.42)
Prejudices are dangerous because they're regarded as easy truth. When people believe in their prejudices (and think that everyone else believes in them, too) the cycle of prejudice can't be broken. But if people expose the Prejudice Emperor as wearing no clothes, well, it might affect some change.
"I want you to show them the difference between what they think you are and what you can be." (24.40)
Grant isn't asking Jefferson to just be better than people expect him to be. He's asking him to teach them a lesson about prejudice by showing them how wrong their expectations are about him. Only if they are made aware of the difference between their expectations and reality will they start to question their prejudices.
"Their forefathers said that we're only three-fifths human—and they believe it to this day." (24.43)
Grant is referring to the Three-Fifths Compromise, a decision between northern and southern states to count slaves as three-fifths of a human being for purposes of taxes and number of representatives in the House. That political decision helped cement prejudices about the worth of black people in the minds of Americans.
Dumb as hell, but prejudiced as hell. They had no other place to go to do their drinking—they would not dare go to any of the white clubs—so they would come here and bring their prejudiced attitude with them. (25.6)
It's kind of hard to understand why Grant uses the word "but" in that first sentence about the biracial bricklayers. Is he really surprised that someone who is "dumb as hell" would also be "prejudiced as hell"? Because prejudices are lazy—they save us time thinking through things and arriving at an informed decision.