"And the first came out red" (Genesis 25:25): for while he was still in his mother's womb, he drank the blood of her menstruation [the source of his bloody, ruddy nature].
The Redness of Esau Already Tainted Within the Womb
Yalkut Shimoni on Torah 110:25
וַיֵּצֵא הָרִאשׁוֹן אַדְמֹנִי, שֶׁעַד שֶׁהוּא בִּמְעֵי אִמּוֹ שָׁתָה דַּם נִדָּתָהּ.