Hi Slimer,
The colour of axolotls is dependent upon pigment cells called chromatophores. These cells are melanophores (containing eumelanin, a black-brown pigment), xanthophores (containing carotenoids and pteridines, yellow and reddish pigments) and iridophores (containing crystalised purines, which impart a shiny iridescence).
Non melanoid axies can have shiny pigments on their bodies. Try looking around the gills and around the tail, in my experience, they tend to accumulate around these areas.
I can see your interest in colour genetics, so i have attached some links for you to look through in your leisure.
http://www.axolotl.org/genetics.htm
http://www.ambystoma.org/AGSC/strains.htm
http://www.ambystoma.org/AGSC/mutantslist.htm
The axanthic mutant is characterized by the absence of xanthophores and iridophores. Melanophores are normal, and otherwise wild-type animals are uniformly dark gray. Dark axanthic axolotls are not vigorous, and Frost reports a susceptibility to viral infections (
Frost et al., 1986). Axanthic animals also homozygous for
albino (a) are hardy animals. Older axanthic albinos are pale yellow, apparently due to riboflavins in the diet (
Bukowski, et al., 1990).
In view of the above literature, i hypothesise that the axanthic gene is 'selected' or 'carried' concurrently with genes that affect immunity, particularly to cell mediated immunity against viral infections.
Regards.