This is an excellent question. Of course, when we refer to tone, we think about the attitude of the author towards his subject matter. What is distinctive about this novel is the simplicity of its prose, and how it is used to show the author's feelings about the injustice of what is going on in S. Africa, and the deep hurt that is being caused to both the land and the people because of the harsh racial schism that divides the country. Consider the following passage:
Have no doubt it is fear in the land. For what can men do when so many have grown lawless? Who can enjoy the lovely land, who can enjoy the seventy years, and the sun that pours down on the earth, when there is fear in the heart? Who can walk quietly in the shadow of the jacarandas, when their beauty is grown to danger? Who can lie peacefully abed, while the darkness holds some secret? What lovers can lie sweetly under the stars, when menace grows with the measure of their seclusion?
Note here how Paton uses a series of rhetorical questions to express his sadness and deep mourning of the fear that his countrymen have and how this is affecting their ability to live and enjoy life. Man, because of fear, Paton argues, is harried and restless, and unable to live and prosper. Throughout his sadness is conveyed.
However, consider how the tone of the novel changes towards the end, especially when Kumalo begins to find new hope with the help of Jarvis. The final paragraph of the story contains the sentence:
Ndotsheni is still in darkness, but the light will come there also.
We are able to see that the novel ends in an attitude of hope - Jarvis and Kumalo form a friendship and Kumalo even befriends the son of the man who his son killed, illustrating the ability of man to breach the unnatural divide that society has imposed in S. Africa between whites and blacks, and the novel ends, if not optimistically, at least offering us the hope of a brighter future.