Squeak comes home, from her meeting with the warden, bruised, and with her dress ripped. Squeak says that the warden has forced her to have sex with him. Squeak tells Harpo and the others that, first, the warden made her talk about her family, and then he asked her to undress. The warden repeated to her that he is not her uncle—he seemed upset at the idea that he could be related to a black woman—and that, since he is not her uncle, their having sex would not be not immoral.
Again, one of the great fears of white society in the South, at the time, was the idea that the races could mix, that the lines separating white from black could be broken down. Of course, many children of white and black parents were born, but, as above, these children were simply called "black," thus avoiding confrontation of the reality that these racial divisions were socially, and not biologically, constructed.
After telling this story, Squeak, who is understandably traumatized by her encounter with the warden, asks Harpo if Harpo really loves her, or if he just loves the fact that she has "yellow" skin (that she is of mixed white and black heritage). Harpo says he loves Squeak, and Squeak replies that, if this is true, Harpo ought to call her Mary Agnes, as a sign of respect.
A scene of violence against women, directed at Squeak, that echoes the violence to which Celie has been subjected. Squeak only does what her family wants her to do, and it does result in Sofia's release into the mayor's family's custody. But Squeak has had to sacrifice herself in order for Sofia's freedom to be gained, and Harpo seems to acknowledge it here when he calls her by her name.