I picked up 2 small veil tails that were the only 2 in the batch that didn’t have any black pigmentation on them. A few months later their finnage started to get dark. Then as it got older it seemed to get really dark in some areas and lose all pigmentation despite good water conditions and low stress.
The first photo is midway, second is adult, bottom photo is at 6”. 3.5” of tail. Lol.