"Iridophores contain crystals, which diffract the light, resulting in iridescence or scattering."

my wild guess would be, since BPs dont show iridescence, that these could be the white ones.

anyway i reckon there must be something, because there is a visual difference between the extremely white piebald white, and the translucent skin of some BELs and lavender albinos, where color from tissue and blood comes through.

some have so translucent skin that as hatchlings you can see the black of their eyeballs through the top of their head. but not with an all-white pied, that white is intransparent. ive also seen cases of white patterning on top of the translucency.

look at the second and third picture here:
http://www.worldofballpythons.com/morphs/leche/ thats white on translucent, also you see the eyeballs shine through the head.

for comparison:
http://www.worldofballpythons.com/morphs/lesser-pied/

But you corrected me on one thing (maybe), i believed there to be a brown pigment that is seperate from the black one. if there is only one, that is brown in low concentrations and black in high concentrations, how do you make medium grey or light grey? but then, maybe the brown is a mix of yellow and black, and if you remove the yellow you get the greys.