Toothless had decided long ago that red was his least favorite color.
Red was the color of fire – fire that burns and injures and kills. The color of the fire that very nearly killed Hiccup. The color of the monstrous dragon who had nearly killed them both.
Red was also the color that came out of Hiccup's skin when he was hurt. Blood, he called it. Toothless hated blood, knowing the sight of it meant that Hiccup was hurt or in pain. He hated seeing his human in pain.
With all of these negative associations with the color red, Toothless had to admit he was (maybe childishly) disappointed when Gobber handed Hiccup Toothless' new tail fin – Toothless' new red tail fin.
But he left Hiccup put it on him anyway, of course; he didn't have much of a choice, as it was his only ticket to flight. He didn't like it, though, all the terrible things the color of his tail fin reminded him of. He tried, multiple times, to get Hiccup to understand his distaste for the color, to get Hiccup to change its color, but Hiccup could not understand what it was that the dragon wanted. And so, Toothless was stuck with his new red tail fin.
If he was stuck with it, he may as well learn to like it, the dragon realized. And soon, to his surprise, he found reasons to like the color. Red was the color of the morning sun that spread across the horizon as he and his human flew through the skies. Red was the color Hiccup's cheeks turned after Astrid kissed him. Red was the color of the hair on Hiccup's head, and the color of the fire that kept his human warm on frozen nights.
Maybe all colors were like this, Toothless realized. Good and bad. Harmful and helpful. Destructive and delightful. Maybe, then, red wasn't such a bad color after all.
Toothless decided, after a while, that he liked his new red tail fin.