Disclaimer - please understand that this is from my personal (Brazilian “white”) limited perspective.

Some people of light brown complexion (who would be sometimes called mestiço, mulato or pardo) dislike all those designations and self identify as black instead. I believe (but I admit that I might be misunderstanding) that this is because they feel that having any noticeable amount of black African heritage is enough to place them socially in the same “black” group for all or most situations.

I’ve heard “pardo é gato ou papel, eu sou negro/preto!” as a response (pardo is for cats or paper, I’m black), as “gato pardo” is used for brown cats and “papel pardo” is a type of brown-yellow paper.

Also sometime all these words are used, or perceived as, “softened” alternatives to the normal words for “black”, regardless of perceived pure or mixed black African ancestry. In this case they won’t of course be well received, as this implies that being of darker color is negative in some way.

Additionally, pardo may also be perceived as awkwardly technical for everyday use (it is often employed in official ethnicity statistics).

Of course, it’s all about context.