Kohaku (紅白) literally means red and white, and the combination symbolizes good luck in Japanese culture. Also, the strands of carrots and daikon are said to look like Mizuhiki (水引), which are charms made from red and white cord tied into decorative knots. These are often used to seal envelopes presented at auspicious occasions and are said to ward off bad spirits while tying people together through a common bond. These, combined with the fact that food prepared for Osechi needs to last for a few days (before the era of refrigeration), makes the inclusion of Kohaku Namasu in Osechi Ryori a no-brainer.