Anerythristic is lacking red pigment. Axthantic is lacking yellow.
Axanthic bps aren’t completely black and white because the brown pigment is still there. However some genes that reduce the brown pigment can make the axanthics more black and white. Because there isn’t really a red pigment in ball python it isn’t something described … corn snakes have that red pigment that can be removed creating Anerythristic. Hypomelanstic just means less melanin, … axanthic isn’t creating more melanin it is removing a color.
BPs do not have red pigment, so they cannot be anerythristic. The pigment responsible for brown is the yellow pigment. Axanthic is actually an incorrect term for the mutations, the proper term would be hypoxanthic. They lack some of the pigment, but still have some no matter the animal. The chance of anyone changing the term is unlikely, despite the fact it has been proven incorrect.
The colour palette in ball pythons is based on the brown (melanin) and yellow (xanthin) pigments.
The reason the “alien heads” appear as a golden/tan colour is because it is a combination of the yellow and brown pigments being present in the same cells/regions. Imagine if you have a piece of yellow stretch film and a piece of light brown stretch film laid on top of one another and you are looking at a bright light through it. The light looks golden. Remove the brown film and the light now looks yellow (Albino). Remove the yellow film and the light looks a light dingy tan (Axanthic)
Hypomelanism is a reduction in the amount of melanin. All the T-pos “albino”-type morphs (Lav, Candy/Toffee, Ultramel, Caramel, Banana, etc…) are, strictly speaking, hypomelanistic. Pastel is also a variant of hypomelanism. What we call Hypo in the hobby is actually a hypo-pigmentation mutant and not a hypomelanistic one
Melanin is a brown pigment. In high concentrations it appears black but that is down to mechanisms of physics and chemistry. In lower concentrations it is any of various shades of brown through tan. In combination with the yellow, your eyes perceive the gold of the alien heads.
Baby Axanthincs look black/grey because of, again, physics, chemistry, and optic nerve excitation thresholds.
None of the “Axanthic” morphs in balls (or carpets or corns or many of the other species) are complete axanthism. It would be more proper to call them ‘Hypoxanthic’. But good luck changing three decades of inertia on that one LOL