A sour cream beaner is a slang term for white mexicans. While ethnically hispanic, skin just happens to be light. May be mistaken for caucasian by people who cannot identify race. Usually are given away to be hispanic because of names that many identify with latinos, such as "Garcia', or "Lopez".
Person 1: Why are you dating a white girl?
Hispanic Guy: She's not a white girl, just a sour cream beaner.