Anerythrism is an autosomal recessive mutation which results in the absence of the red pigment erythrin. As a result, these boas are grey, white, silver, and black.
Chromatophores are specialized pigmentation cells found in the dermis and epidermis of snakes. The combination of pigments expressed by melanophores (black pigments), xanthophores (yellow pigments), erythrophores (red pigments), and leucophores (white pigments), gives snakes their coloration. Erythrophores synthesize enzymes that convert carotenoids into the red pigment erythrin. The anerythristic mutation is the result of non-functional enzymes, thus erythrin is absent or decreased.
There are two variations of this mutation that will result in an anerythristic boa. The first is referred to as a type one mutation which originates from Colombian boas. This is the most common variation of the mutation, these animals have increased yellow and brown pigmentation with age. The second is referred to as a type two mutation which originates from Nicaraguan boas. This mutation is more desirable as it produces darker animals. I believe that type 2 anerythristic boas may be axanthic (absence or reduction of yellow pigmentation) as well as they have almost no yellow coloration. Xanthophores synthesize enzymes that convert carotenoids into the yellow pigment xanthin. The axanthic mutation is the result of non-functional enzymes, thus xanthin is absent or decreased.