I'm not sure exactly as I haven't read it yet. But generally, a 'Wings of Fire' graphic novel might be about dragons and their adventures, with 'indigo' perhaps referring to a specific character or aspect related to indigo - colored dragons or elements in their world.
Since it's a 'Wings of Fire' graphic novel, it might feature the typical dragon characters that are common in the series. However, the 'indigo' in the title makes me think that there could be a dragon with indigo wings as a central character. There could also be other supporting characters like friends or foes of this indigo - winged dragon. Maybe there are dragonets or elder dragons who play important roles in the story as well, guiding or challenging the main character throughout their journey.