The mode of inheritance seems to be polygenic and can occur when a genetic red animal is bred to a non-red unrelated one. 
Madam Blueberry is believed to have started the red line of coastal carpets.
Erick Hernandez has been working with MBB red since 2012. He has done red x red, outcrosses, and backcrosses. He believes that red is a polygenetic hypomelanistic trait. 
The MBB red animals are believed to be a hypomelanistic trait with varying degrees of expression. There seem to be levels of the red pigment expressed from coffee-colored animals to highly extreme red animals.
With age, animals that fully express these genes tend to lose their black. Like a reverse IMG Boa. 
No known proven lines
No known related traits