She's white and also looks soft, squishy and fun to squeeze but I agree that does NOT make her a marshmallow! I still affectionately refer to her as "the marshmallow mare" sometimes though, but that's only because practically every other brony calls her a marshmallow. That is all
its because in one episode ( cant remember which one, Suited for suscess maybe?) she reached her hooves towards the camera and the camera angle made her legs look like marshmallows. I dont always call her a marshmallow but when i do its used as a term of affection, not an insult.