Because You're Not Worth It
Hair dye is a pretty obvious symbol of an identity crisis. If a character doesn't want to cut off all of his or her hair, then hair dye is the next best step. What you might not notice is that both Libby and Ben resort to hair dye to try to become who they want to be. Ben dyes his hair black, to go with his dark goth personality. That's more who Diondra wants Ben to be, but he goes along with whatever she wants, anyway.
Libby, on the other hand, bleaches her hair blonde. It's like light and dark, good sibling vs. bad sibling. But as Libby gets closer to finding the real killer—and reconciling with Ben and her past—she dyes her hair closer to its natural color, "Scarlet Sass" (28.4). She's embracing the Day identity once again. At this point, she's using it to her advantage, hoping that by reminding others of her mother, they'll spill the beans about what they really know. But we have a feeling that she'll keep the color after the book ends.