Shug, Mr. ____, and Celie dress Squeak up to resemble a white woman, applying extra make-up to her face to lighten her skin. They tell her to tell the warden that Sofia doesn't really mind her punishment, and that the only thing Sofia would hate would be to serve as maid to a white woman. Mr. ____ and Shug believe this lie, an instance of reverse psychology, will cause the warden to do exactly that—to release Sofia as a maid to a white family. This release, Shug and Mr. ____ hope, will save Sofia's life.
This use of reverse psychology is rather ingenious, and it is effective. Celie and others in the family, especially Shug, understand the kinds of fears that motivate white society in the South. One of those fears is a total anxiety about the status of black people in the community, whom white authorities know they are holding down. This anxiety becomes a kind of cruelty, and if the wardens believe Sofia is not being punished enough, they will seek to increase that punishment.
Buster says this sounds like a bit of "Uncle Tomming," or behaving in a servile fashion to white authority, but Shug jokes to the family that, after all, the warden is actually Squeak's uncle to begin with.
A reference to the novel Uncle Tom's Cabin, by Harriet Beacher Stowe. Uncle Tom is a slave, who, despite experiencing brutal treatment at the hand of whites refuses fight back. When Uncle Tom's cabin was written Uncle Tom was meant to serve as a symbol of the power of Christian goodness, but by the time in which The Color Purple is set the character had come to be seen as a man who was complicit in his own enslavement by refusing to stand up against injustice.