Kill them all!
Henzell says...
  1. A group of resistance-related scales mutations. Includes molten scales (rF+), icy blue scales (rC+), yellow scales (rCorr), green scales (rPois), or metallic scales (rElec). The resistance is gained at mutation level 3. Different colours have different levels of AC, and some colours don't give a resistance.
  2. Molten [2, 3, 4 rF+]; Icy [2, 3, 4 rC+]; Yellow [2, 3, 4 rCorr]; Green [2, 3, 4rPois]; Metallic [2, 3, 4 rElec]; Iridescent [2, 4, 6] Bone Plates [SH+4, 6, 8]; Rugged Brown [1 +3% HP, 2 +5% HP, 3 +7% HP]; Repulsion [EV+2, EV+3, EV+4RMsl]; Sturdy [ER-2, ER-4, ER-6]; Sanguine [6, 9, 12 at <67%hp]; Sharp [1 Slay+1, 2 Slay+2, 3 Slay+3]

Information based on master branch and the great LearnDB.
© Dungeon Crawl Stone Soup, 2010.