Board Thread:Questions and Answers/@comment-26952946-20160609150654/@comment-25495690-20161116070544

Green is the mode and color of the  Green Human, who is also associated with  Green Attacks, shielding,  frying pans, cooking, healing, and kindness.

-Undertale wiki on Soul Modes

A cleric tank will take the hits, heal the party, and of course is a cook (because everybody knows chefs are natural born pacificts), i.e. most healing items in the game are Food. Undyne just wants to protect her people and give them freedom, even if that means sacrificing another filthy human! (they got so far, it'd be crueler not to go through... right?)