Proof that inside spiral zetsu it isn't yamato could be this. This is picture of yamato right?
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Now look at his panel where suigetsu shoots spiral zetsu with a water technique
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Now we all know that Manga is black and white. So what is shown in black usually is black. Above we see spiral zetsu's hair to be black not brown like Yamato's.
Just putting it out there, as one who used to believe it was Yamato inside im pretty upset