When Rumpelstiltskin an Nimue became "the dark one," they both changed their look - specifically their skin. They had a greenish skin tone so that they resembled more of a "crocodile."
But when Emma and Killian became the dark one(s), they didn't change their look the same way as Rumpelstiltskin an Nimue did. The most noticeable change was Emma's hair color.
Why was that?