In the end, when Minnie's friends exult that "there's not a negro on the square. Not one," the reader can gather that it's because the African-Americans in town understand that their race is considered a crime, but that murdering them is not.
Conversely, Minnie Cooper's whiteness is enough to prove to the mob that she's telling the truth -- even though no one knows what she said or whether she said anything at all. The "youth" in the barber shop talks about the importance of taking "a white woman's word" before that of an African-American man, and he is offended that Hawkshaw, the barber, would "accuse a white woman of lying," as if race, gender, and truthfulness are inextricably linked.