I'm just throwing my two cents in here, but being an albino as I am, I've read up a lot on it and there are many different forms of it, for instance I have both ocular albinism in my eyes and type 2 albinism, as some refer to it, in my skin. Albinism is, as CessBee said, a genetic mutation. You can carry the gene for albinism with a heterozygous fashion as both of my parents did and not know it, just your daily lesson. :P
I don't know a whole lot about albinism in horses, but in the 4-H book that I have it says there is an albino horse registry.
To get back on topic, common colors near me are chestnut and bay, but mostly chestnut and rare colors are perlino, grulla, cremello, palomino, and paints other than chestnut.
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