Why does "pink belly" even happen?

im not 100% sure, but i think white exists as a pigment in BPs, and when you get high skin transparency with low pigmentation of any kind you get into these light lavender and pinkish hues. that with increased bloodflow.... pink.

but im not entirely sure, and cannot be without pictures of the belly in question.

for an example of what i mean with white pigment, take a look at the second and third image for this morph:
http://www.worldofballpythons.com/morphs/leche/ and look at the head, how the darkness of the eyes shines through around and on top of the eyes.