James,
 
I am not really big into cowries, but like everyone, I find them beautiful
and I have most of the more common ones and a couple of the standard
rarities.  I have several C. tigris just because of the characteristics you
have noticed -- a lot of variety.  A really black C.tigris is indeed a bit
rare and commands a hefty price.  I would think one as you describe that is
not all black but heavily melanistic is certainly worthy of display in your
collection.  If nothing else it makes a nice contrast with other C. tigris.
This is an extremely variable species, from white to normal to black, and
all manner of variations in between.  As for the yellow one, I don't know.
The whole business with C. tigris is wild.  You can find really great
looking specimens in bins in shell stores for a couple of bucks and at the
same time a little color change can make the brother, sister, or cousin of
that $2.00 shell worth hundreds.  And it is all the same species.  We
probably ought to be thankful that such a large, well colored cowrie is
common enough that every shell collector can have a nice representative of
the species.
 
Tom Eichhorst in New Mexico, USA