I think in those cases that's mostly because Mexicans and African-Americans are two large, fairly homogeneous non-white groups traditionally concentrated in certain parts of the country. But as far as I can tell white, black and Asian teenagers in San Diego, California are not radically different from white, black and Asian teenagers in Providence, Rhode Island. Mainstream Southern culture probably does have some black influences, though. Anyway, I think we might have to properly define culture first, because the equation obviously changes when you include popular youth culture, where there seem to be more non-white cultural influences...but even then we would have to decide how we define cultural expression by 'Westernized' non-whites. It's all pretty muddy, actually.