A touch of gray may suit some people, but many others are grateful for dyes that return hair to its natural color. For example, black dyes made with small organic molecules are popular among Asia’s graying population, but these products can be problematic, say researchers led by Jong-Rok Jeon of South Korea’s Gyeongsang National University. For instance, many dyes use p-phenylenediamine for its ability to penetrate hair and help provide long-lasting color. But the molecule is also a skin irritant that may lead to hair loss, the researchers explain. They thought dyes using polydopamine particles would be safer alternatives because of the synthetic polymer’s similarity to natural melanin pigments found in dark hair. Pure polydopamine does dye human gray hair samples but not to the deep shades desired by Korean consumers, the researchers found. They suspected that getting more synthetic melanin from the liquid dye to solid hair strands would help gray hair go back to black, and they found iron was especially effective at recruiting polydopamine. Iron chelates polydopamine clusters with one another and appears to bind the polymer’s amine and catechol groups to hair’s keratin (ACS Biomater. Sci. Eng. 2017, DOI: 10.1021/acsbiomaterials.7b00031). The resulting black dye can withstand repeated washings, and mice exposed to it daily for 16 days all lived and kept their hair—unlike most test mice colored with p-phenylenediamine dye.