Irene is articulating one of the major themes of the novel here. Echoing what Brian says two chapters previously about the indefinite definition of race, Irene conveys the ambiguity of race to someone outside of her own. The fact that the identification of race is ambiguous and unclear when in conversation with somone of another race speaks volumes to the fact that race is more than color. It isn't so simple that Irene can point to the difference of skin tone between her and Wentworth and say "that's it." Larsen uses one of Irene's few memories of the night to reinforce the concept that the dynamic of race goes deeper than skin.