Board Thread:Fun and Games/@comment-30829949-20161111084235/@comment-27701762-20161111093247

This doesn't really work, since you would need to explain the green and yellow souls. But neither Undyne nor Alphys ever say that they've met a human before or hint at such. And there wouldn't be any other source for them to be able to acquire that power.

The underlying idea isn't really needed, though. Undyne simply has the power to turn people's souls green, as Papyrus says. Presumably it is an effect of magic on the soul, just as other forms of magic have different effects upon the soul. There is no reason that the logic cannot be extended that other monsters have the ability to affect the color of souls, and that Muffet can turn them purple, Papyrus/Sans blue, etc., etc.