Cats with brown coats have a brown gene variant that reduces the amount of black pigment. While the brown tabby is easy enough to find, chocolate and cinnamon cats are rarer.
Melanin, which is a type of pigment. Melanin has two structural components, eumelanin (shades of brown and black) and pheomelanin(fur, red and yellow coloring).
Black cats don’t have a brown gene, the rusty discoloration of the fur can usually be blamed on sun exposure, visible on long-hair breeds like Maine Coons.