Hmm...come to think of it...I heard the "milk makes white" comment as a kid....and then I think I heard the chocolate milk comment much later....maybe in my teens or later than that. Ah well.
There's no logic behind it....it's so ridiculous that I find it funny when I look back on it.
Later than your teens?! Heaven forbid you drank any black coffee in front of them, they would have had a field day. Crazy people.