How does Nimona’s visual characterization support her evolution throughout the book?

Throughout the story, Blackheart and Goldenloin’s trajectories are closely intertwined (e.g., training together, turning into nemeses, reuniting through a common cause). What narrative choices put them at odds and/or draw parallels between them?

How does Nimona’s moral code evolve throughout the story? Looking back at the first part of the book, how does her earlier behavior hint at her deeper nature or foreshadow any changes?