Black coat color is produced by a pigment called melanin. Melanin is the same pigment in human skin that turns our skin brown(er) when we are exposed to sunshine. Melanin is found in higher concentration in black coated animals than other color coats.
Melanin is made from amino acids obtained from the diet. Amino acids are smaller chemical structures that make up the larger structure called protein. Phenylalanine and tyrosine are two important amino acids involved in melanin synthesis.
Most mammals can produce tyrosine from phenylalanine. However, cats are not as able to make the conversion as other mammals.
Therefore, if a black cat is eating a diet low in animal sourced proteins (beef, lamb, pork, fish, chicken, eggs, dairy), that cat’s coat may turn reddish. The red color almost looks like the coat is bleached from the sun. However, healthy coats do not bleach from the sun.
To treat the red coat – increase meat in the cat’s diet. This can’t be as simple as switching from a grain-based food to a meat-based food. Even better would be to start your cat on a real meat diet – either raw or very lightly cooked.