I took my kids to a bounce house place. I met a friend and her kids there. All our kids were running around together with another little boy we didn't know. This little boy was black.
At one point they all ran up to us and were jumping and being silly. This boy was facing my friend and DS1 touches the back of his head and says "Mommy his hair is funny!"
I tried not to look worried or embarrassed and told DS1 that yep, his hair is brown and curly and yours is straight and blond. Now go play!
He's never mentioned anything about noticing differences in hair type or skin color. So this was a first.