This is a rather dark novel through the first two thirds of the text, but in the last third of the book the mood lightens. Though the mood does not equate directly with the tone, it does influence the overall tone of the novel.
This is true, in large part, because Celie's letters are so full of bitterness, which is an element of the novel's tone. Hardship after hardship and abuse after abuse characterize Celie's letters.
Written in a hybrid-grammar mixing elements of vernacular and "proper" grammatical forms, the text presents a tone of folk-oriented intimacy, focused on dark subjects and eventually on personal growth.
The tone then is folksy and hopeful and also bitter and defeated - a conversation on losing almost everything then gaining unexpected poise in the end.