I have used Jet Hot High Performance Coatings or Jet Hot Coat
https://www.jet-hot.com/ cast iron color on my LS6 and other show cars and the results were superb. The jet Hot coat process dissipates heat quicker too. After three years of running on my LS6, there were zero signs of discoloration. What do you expect from a company who also coats exhaust systems for NASA. Jim