The Color Purple is an epistolary novel, which means that there is no external narrative tone present to provide commentary on characters' actions or contextualize those actions within a broader societal narrative. Celie is our primary narrative voice, given that she is our primary letter-writer. Nettie will occasionally weigh in as well, through her letters. As such, they are the lens through which readers experience any additional tonal meanings Walker intends to imbue in the text.
Both Celie and her sister begin life fairly isolated from the outside world, born into rural poverty in the American South. As such, the tone both women strike is often one of surprised discovery. This surprise reflects back on the reader and undermines certain cultural practices and assumptions, forcing the reader to question that which might have, up until this point, gone relatively unquestioned. Take, for instance, the following excerpt from Letter 55:
Oh, Celie, there are colored people in the world who want us to know! Want us to grow and see the light! They are not all mean like Pa and Albert, or beaten down like Ma was. Corrine and Samuel have a wonderful marriage.
In this passage, Nettie expresses shock at the fact that there are Black people in the world who support their own, rather than tearing them down. Walker uses Nettie's tone of surprise to undermine common assumptions made by community outsiders. Despite the efforts of the white supremacist state to tear apart Black families, Walker asserts, those families are in the process of healing and coming back together. The community is not divided and toxic on the whole; rather, Black people are doing the hard and necessary work of healing from generational trauma and systemic racism.