poly's change color when you change the subtrate. Mine do the same thing. You will eventually see him become a little darker with the black subtrate. For some reason switching from whit to black the lighten then in a few days eventually become darker.
I think most want to see a pic after he's been on black gravel for a few days to make sure he's for sure color morphing.
I'm confused whether to call him Leutistic or not. But I want to bring up something The Trust brought up today in "Albino Clown Loach" in Photo Lounge. He said that if you breed albinoxnormal you'll get a fish that is part albino and part normal. Might be what you have.