You might not even want to bring up the idea of racialized differences this early. Maybe "there's people of all shapes and sizes and all of their minds have surprises".
When I was 6 I heard the phrase "Black person" and had no idea what it meant. I had seen plenty of people with dark brown skin, but they weren't really a category until my 1st grade teacher (who is Black) spelled it out for me. I did have a fairly sheltered, well-off early childhood though.
Maybe teaching your kid about how white skin can change colors from the sun is a useful thing to do, showing that it's mutable.