Hi James,
I think I heard that Papuina (Papustyla) pulcherrima grows yellow during
drought conditions?  Or something like that, I'm not certain??  However, in
the case of your specimen, the normal green color on the columellar area is a
dead giveaway - the shell has been bleached.  The bleach was unable to reach
the green pigment on the interior surface because the pigment was overglazed
there with a thin layer of shell.  I have seen this on land snails with
colored periostracum.  The bleach removes the periostracum from the shell
exterior, often leaving the shell pure white.  But the normal periostracum
color and pattern remain on the columellar surface because there, where the
shell is in direct contact with the soft tissues of the snail, the
periostracum is overglazed with a thin, transparent layer of shell.
Paul M.