She also teaches (through her actions and Atticus' respect) the Finch children that skin color does not matter. Scout witness the difference between black and white in Chapter 16 when Atticus and Aunt Alexandra disagree about speaking openly in front of the "colored" help.Calpurnia teaches Scout many of the same lessons. Since Atticus sees Calpurnia as a member of the family, she is allowed to freely scold and lecture Scout if need be. Cal teaches Scout about treating people with respect and taking time to understand others. Scout watched Cal and even sees that being a girl isn't such a bad thing. Cal, despite being black and the Finch's cook, becomes a kind of mother to Scout in many ways.
First, Cal acts as a mother figure for Scout. For example, after Cal gets after Scout for how she treats Walter Cunningham early in the novel, Scout is angry at her. However, when she returns from school Cal is nice to her and cooks her some crackling bread. In this way she is mothering Scout and disciplining her at the same time.
Second, she acts as a counterpoint to Aunt Alexandra. In the previous example, Cal scolds Scout for embarrassing Walter at lunch. Cal emphasizes to Scout that while she might be of a higher social class than Walter, she still must respect his wishes.
It is no coincidence that later in the book, Alexandra makes an entirely different point. She tells Scout that she is too good to socialize with Walter; Finches don't socialize with white trash. Because of Cal's influence, Scout is able to see both sides of the issue and come to her own conclusion.
Cal also serves to show Scout a different side of Maycomb society. Look at all that Scout and Jem learn when Cal takes them to her black church. She realizes that Cal exists in two different words - a black world at home and a white one with Scout's family.