I posted a thread about this back in 2010 here:
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If you look, their faces look absurdly alike. I was actually going to add to it because on one of the new anime episodes when madara rips of his arm, what should be blood is white. But it's no where near as simple as madara being a zetsu clone.
Madara took the 1st cells, and grew the giant lotus from them-- the same lotus he used to replace that missing arm. But it seems as though he used that in order to combine his with the 1st cell lines, meaning that zetsu, which at least the white ones being grown from the lotus, is more technically a clone of madara grown from the 1st cells.
Now if he's like the clone zetsu made for kisame, idk. But it seems clear at this point that he is certainly made at least partly out of the stuff of zetsu, and that's why he hides his face. Because he has the same face as zetsu, if not half-black and white.