Here is a lengthy section from a paper I wrote years ago on the topic, which doesn't answer the question of how-to research those ancestors but defines the context of the definition - cross-culturally:

COLOR, CASTE and CLASS in the AMERICAS

Studies of slavery both here, in the Caribbean and Brazil highlight the development of a separate group of mulattos. Degler focuses on the history of differing developments of slavery and race relations between the U.S. and Brazil, counterpoising the rigid racial segregation practiced here, in opposition to the "mulatto escape-hatch" provided in Brazil. (Deglar, 1971) However he fails to appreciate that we had our own mulatto escape hatch here as well. Brazil's allowed for an escape into the greater (whiter) society, whereas ours propelled the mulatto to the top of the black heap, trapped in a world where one drop of black blood defines you as black ad infinitum.