I disagree, homogeneous societies are less tolerant of other races. These societies have little experience with other races so when they are exposed to different races the people can react with xenophobia and hatred. Homogeneous societies tend to be more rigid and less open to change and different cultures. This is why racism is more prevalent.
I am not sure it makes much of a difference because people tend to be turf oriented and intolerant of those who are different from them, but it would seem that if a culture is less diverse than it can convince itself of its superiority and be less tolerant of other races.
Yes, homogeneous societies are more tolerant of other races, because they find ways to keep other races out. A heterogeneous society is used to having and dealing with lots of different types of people. After a while, most people just go about their business and don't think about it. In a single-raced society, anyone who is different stands out.
America is a pretty clear cut example of "melting pot" societies having their race issues too, but a homogeneous society by definition is going to be less accepting of other races because it's going to have even less experience with them. People aren't naturally trusting, they're not going to be receptive to something unfamiliar.
Japan is a relatively homogeneous culture, and in some ways
its people seem less tolerant of other races. Yet the United States of America
is a relatively heterogeneous society that brags about its racially mixed heritage,
and America is sometimes torn by racial tensions, as when a person of partly Mexican
descent shoots a young man of partly African descent and gets away with it. It could
be that racism is not a social characteristic but a personal one. Nevertheless,
it is no doubt difficult to free oneself of prejudices unknowingly absorbed
from a homogeneous culture.