Sensing an opportunity, I told him, "You see, until 1957, the entire world was black and white. When I was a kid, everything looked just like this. Color was invented when I was four, and it took quite a while for the world to change. There were times when things were only partially in color, and sometimes they switched back and forth until things stabilized."
He asked, "Then why didn't these pictures change?"
"That's because the ink had already set and dried."
He nodded as he tried to assimilate this new information. A week or so later, after visiting his Mom (my ex), he said, "Dad, Grandma explained it to me...about color and black and white." We shared a laugh, and he has a tale to tell his kids.