According to Leonard Hill in his book "The World's Most Beautiful Seashells",
Papuina pulcherrima  is not rare at all, but "was placed on the United States
endangered species list when a survey team could not find a single live
specimen. The study was conducted during the short dry season when most tree
snails go into aestivation so they can conserve moisture. Many hide under
loose bark, in tree crevices, in epiphytic plants. or similarly well concealed
locations, so their detection is nearly impossible...........In Papua, New
Guinea, they appear by the basketful in local markets. Yellow and white snails
are found in the same population as the green snails."  There is a very good,
bigger than life color picture in this book, too, of green and yellow ones.
 
P.S. - I'm a new subscriber on the CONCH-L . I started collecting shells when
I was 11, mostly from a little storefront in NYC,  (anyone remember Veronica
Parker Johns?) joined the NY Shell Club, and was fairly active for about ten
years.  That was a long time ago, and having finally settled down in one
place, I'm ressurecting my interest in shells and feel a bit like Rip Van
Winkle - half the scientific names in my collection seem to have been changed!
I have one of these little green snails and it  has always been among my
favorites.