The Book Thief
Death's fascination with the colors of the sky functions as imagery. It helps cast the mood of the story, and creates much of the atmosphere. By focusing on the sky-colors at the times of human deaths, Death suggests that there is a connection between a person's death and the natural world. The idea that each person dies with their own color of sky presents a vision of a universe which cares about humans, and isn't indifferent to them. For Death, the colors are also edible, and he sucks on them for distraction while he's on the job.