Erythristic is refering to red pigmentation (erythrophores are the red producing chromatophores), so technically anerythristic means lacking red pigmentation. Xanthic is refering to high amounts of yellow (xanthophores are the yellow producing chromatophores) so axanthic is lacking yellow pigmentation.
Thanks again, Evan. I was wondering about that for a while in relation to why black and white Boas are called anerythristic and black and white BP's are called axanthic.