the term "free person of color" was a term that was applied to Pre-Civil War Americans who were viewed as having a degree of non-europeon ancestry who were not slaves or Native Americans. Terms such as "free negro", "free mulatto", "negresse libre(LA), "mulatre libre"(LA), f.p.o.c. applied to persons of African/Europeon, African/Native, as well as persons who were products of complex mixtures of all of these ancestors generationally.
In a nutshell: Anyone who had some "color" to him/her who wasn't a slave or Native American.

"Race" and the classifications which derive from it have no scientific basis in reality anyway. Its all folk-based. Also inconsistent because the classification for one person fluctuated from document to document.
For example, my grandmother descended from a long line of ancestors who were always classified as "mulatto" in French Louisiana up to the mid 2oth century. Others were called "White" too. The death certificate of my grandmothers aunt lists her "race" as White.

Another example is the ethnic identity of Crispus Attucks. Contemporary scholarship and history books call him "African-American" or "Black" even though eyewitness accounts of people who knew Crispus described him as being a "mulatto" and an "Indian". He is never called a "negro".

Something to think about.