The dark (or not so dark) layer is quite superficial, not far from surface. But the vit D production happens deeper in. For sun rays to get there, that layer mu

The dark (or not so dark) layer is quite superficial, not far from surface. But the vit D production happens deeper in. For sun rays to get there, that layer must be light-colored. Sure, some photons do get reflected, but the benefit is from those photons that get deeper into the skin.