Red was a color that Zuko knew he would never be able to forget.
It was the color that symbolized the land that he had been birthed in; the land that he was banished from while still a child, unable to fully grasp the gravity his situation. It was the color of the country that he would one day rule.
It was the color of his mother's robes; the shade of the residue left on his forehead when she kissed him. It was the last color he'd seen of her when she left.
It was the color of his element: red flames, heat, and fire. It was the most prominent of the many shades he saw when he be bent the flames to do his bidding.
Red was the color of the blood that stained Katara's lips when she told him that she loved him for the first and last time. It was the color that he had seen when he had cut down her attacker.
Red was the color that he could never forget.