I've owned two grays that are/were almost entirely white, and they really shine (in their summer coats anyway). So I'm pretty sure it's NOT the hair color. But they have dark skin underneath, so I don't know if that has anything to do with it.
I did have a Paint that was almost entirely white, and now that you mention it, I really don't remember him being shiny like my grays.
Maybe the pink skin under the hair on a Paint gives them a more dingy look or something. (I think the dark skin on a gray gives them a slightly bluish cast, especially when wet). But the hair on my grays has truly been shiny.
What does "alfamo" stands for? Is it something you can add a little vegetable oil to? I do give my guys a small pour of vegetable oil over their grain-based feed.