How do the concepts of "biracial" and "multiracial" relate to W. E. B. Du Bois's notion of a "color line"?

Judging by the fact that united states is made up of individuals from different continents and countries. Except the native indians who were the original owners of the land. We can take a very close and yet critical look at number of individuals in the US, above 7 milion, promptly consider themselves to be having double or multiple racial identity personality