True. Not true.

As a society where white is dominant it is true that it is impossible to be racist against whites on the societal level.

Not all societies are white dominant. In them it is absolutely possible to be racist against whites.

As an individual within a white dominant society it is possible to be prejudice against white people, discriminate against them, or act antagonistically toward them. While the definition says the behavior is typically directed towards a marginalized or minority group, typically is not an absolute.

Further, the term also means the belief that different races possess distinct characteristics, abilities, or qualities, especially so as to distinguish them as inferior or superior to one another. So any person regardless of what race they are who is categorizing people based on race, even if they are elevating another race above their own, is behaving as a racist.

Though, at an individual level, “racial bias” may be a better term.