The Melanochrome is the 13th of the 19 genetically-engineered gene-seed organs that are implanted into a Space Marine Neophyte to produce a new Astartes. This hormonal implant is attached to the human lymphatic system and controls the amount of melanin in a Space Marine's skin. Exposure to high levels of sunlight will result in the Astartes' skin naturally darkening to compensate. It also protects the Space Marine from other forms of electromagnetic radiation. Different levels of radiation cause variations of skin color in different Chapters due to mutations in the Melanochrome organ's gene-seed. This can be related to the unusually pale skin of the Blood Angels and the Raven Guard as well as their Successor Chapters, and the dark black skin and red eyes of the Salamanders.
- Codex: Space Marines (5th Edition)