If I'm describing "una persona," but someone who happens to be male, is the correct adjective agreement "una persona famosa" or "una persona famoso?"
Persona is always feminine, and corresponding adjectives must always agree with that gender:
- Juan es alto.
- Juan es una persona alta.
- La persona, un hombre llamado Juan, es alta.
- Juan es una persona alta. También es simpático.
Notice in the last one, in the first sentence I define him as a person and use a feminine adjective. But the next sentence I'm back to talking about him as himself, and revert to masculine.
In Spanish, the adjective always agrees with the noun it describes. The noun "persona" is always feminine. Therefore, all adjective that describe it are feminine too, even if the overall phrase ("una persona famosa") describes a male individual. The adjective is describing the noun "persona", and not the person who the phrase refers to.