If a fish lacks melanin (such as a fader) it is amelanistic. Amelanism is a recessive trait, therefore for a fish to exhibit this trait it must be carrying two genes for the trait. (homozygous) In fish, amelanism is caused by a loss tyrosinase function, just as it is in mammals. Having said that, with fish there can be other pigments present so amelanistic vertebrates are seldom white with pink eyes like amelanistic mammals. The appearance of an amelanistic fish depends on the remaining non-melanin pigments.
Just as it is in albinism, amelanism results from inheritance of recessive alleles for this trait. It is basically the same principal as an albino defect, but with amelanism it is a lack of dark pigment, as opposed to a lack of all pigments, such as in albinism.
Certainly this trait can be further manipulated by FH breeders to produce varying results in gene expression in different strains of fish, but the info that I posted in the above link was only explaining the "fading" gene, and how it originally came about in flowerhorns. What some breeder in Bangkok does with each strain of fish that he produces is anyones guess.
As far as the fading gene itself, there is no mystery.