Sforno on Genesis 25:30/ספורנו על בראשית כ״ה:ל׳

1על כן קרא שמו אדום. When he saw that Esau was so totally absorbed in his futile occupation, an occupation which does not represent the task of man on earth, that he could not even identify the lentils by their name but referred to them only by their colour, he called him by that name himself, i.e. אדום, in the imperative, meaning “paint yourself red by swallowing the red dish!”