Avian crest colormorphs (Patreon)

Avian brights produce three skin pigments: the usual blackish brown, cyan, and yellow. Normally the colors come in seperated bands of yellow and blue, with some difference in spacing, ratios, and intensity. But, there are some disorders and genetic conditions where the coloring is disrupted, the image aboveshows the extreme versions of each.

Blue distribution is mostly genetic but yellow is notably effected by diet, and brights who don't eat enough of certian foods can have pale uncolored skin patches where their yellow is supposed to be. Eat too much of some foods and your blue will start to turn green (a very unattractive color.)

Blue production is technically linked a vital mineral, cobalt, but the pigment is stable enough that after the body produces it there's not much turnover. You could hypothetically deny a bright of cobalt during puberty to induce a yellow crest, but they might die of anemia since they also need it for their blood... so not really that diet-influenced.