What race is Tiger Woods?
What race is Eninem?
What race is the child of mixed parents?
What race is Obama?
It has nothing to do with who they identify with and everything to do with their physical characteristics and color is one of the physical characteristics.
But it's not usually the color. it's real or perceived stereotypes about "how those people act". Their culture.
As I've said, color is one of the physical characteristics by which one's race is determined. How would a racist determine who to be racist against if not by their physical characteristics. They aren't asking people "how they act or live". They make those determinations based on what they look like.
That's not true either because we were hated long after we ceased being slaves. Slavery caused many whites to re-examine their belief in their superiority and the MECHANISM of slavery was a reaction to the reality that their superiority did not exist.
Census is the only reason I can think of