Do you think if the Beta Trolls lived long enough, their skin would turn black like their Ancestors? Or black skin is only exclusive to Ancestors?

Black Skin is actually a stylistic choice to depict Adult Trolls. We can see this same stylistic choice being used on Trizza at the end of Act 1 of Hiveswap.

This is proved in canon by this panel, in which the Handmaid (Slightly Off-Screen) is shown to have Grey Skin, even though she’s depicted completely black in every other panel.

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