As much as I like the original, given the Sans, Papyrus and Gaster so far have very different color palettes so I would match that by going to a new one as well. I like the pinkish red color if you're going for a softer Undyne. The original red accents the original blue a lot making her a stand out character.

If her personality is more docile I would give a lighter and more consistent palette while if it's still as spontaneous try a bolder more solid base but have the stripes and other features a far different color than the base.



Maybe a greener base with accents a brighter green or possible slight yellow.

Or another yellow orange base with a bright base but you could mix in a tip color.

(I'm just googling fish and looking at pallets)