In an American context, mixed race people passing for white would tell others that their darker coloring was due to a Black Dutch or Black Irish ancestry, rather than an African or American Indian grandparent.
As for the Melungeons, researchers have found genetic markers from the Middle East and the Mediterranean in some of them. Middle Eastern ancestry is not found among a surrounding population of British Isles, West African, and American Indian origin. It is probable that at least some of the stories relative to Turkish or Portuguese origin are true.
Also when the Dutch moved in to South Africa there was already a tribe of Cape Coloreds who were part Portuguese and part Hottentot.
"In an American context, mixed race people passing for white would tell others that their darker coloring was due to a Black Dutch or Black Irish ancestry, rather than an African or American Indian grandparent."
It kept them from being listed as "mulatto" or "colored" on the census, which could and often did lead to being run out of the county or having your land seized, in Ol' Virginny at least.
Sort of like the Coneheads explaining their appearance by telling people that they are from France.