You've actually got it backwards. Mestizos were of mixed Native and European heritage, while Mulattos were of African and European Heritage. Spain and Portugal had what were called Casta Paintings to easily convey the potential racial outcomes and their relative rankings in colonial society.

This wikipedia article should give you a basic idea of how the Casta system worked.

Mulatto is mixed European and African. Mestizo is mixed European and Native.

dumb question. smh