There are various theories on why species living in darkness may have
pigments that are capable of reflecting colored light, some of which
we have already heard.  Perhaps these life forms evolved in shallower
seas, where light and color existed, and the pigments served a useful
function back then.  The transition to abyssal habitats may have
occurred later, in which case the pigments are simply vestigial
remnants with no current purpose, like the human appendix.  The
problem with that theory is that the colors and patterns of modern
shallow water organisms should still be serving the useful function
for which they originally evolved.  That seems questionable.  It is
interesting though that most species of animal life found in
subterranean caves are without pigmentation.  There are pure white
fish, crustaceans, insects, molluscs, and various other forms.  Why
hasn't the same sort of thing happened in the dark abyssal ocean?
Beats me.
Another theory is that shell colors are simply accumulations of
certain metabolic wastes.  In other words, color is not the purpose
of the pigments, just a by-product.  The problem with this idea is
that many species show a broad range of color and markings even
within one population.  Do the lighter colored specimens produce less
metabolic waste than the darker ones?  Do albino specimens produce no
waste materials at all?
Akin to that theory is the idea that the colored substances do serve
a useful purpose, but a purpose unrelated to their color, and the
color is, again, simply a by-product.  This has been mentioned
recently, relative to the possibility that some pigmented substances
may strethen certain parts of the shell.  This is like the question
"why is blood red?".  If you approach this question with the idea
that redness itself is an evolutionary advantage, you may come up
with some interesting theories, but you won't find the answer.
Because blood did not evolve "redness" - it developed
oxygen-transporting capacity.  It just so happens that oxygenated
hemoglobin reflects red light, but that fact is entirely incidental
to the animal's welfare, or to the purpose of hemoglobin.  The same
may be true of shell and other animal pigments.  They may serve
various functions, and their light-reflecting properties (that is,
their color) may be entirely incidental and without any specific
purpose at all.  In the same vein, we might ask why so many species
have brown opercula.  Is brown more advantageous than other colors?
Does it serve to camouflage the foot?  Does it blend with the color
of the sediment?  Of course not!  Toughness is the requirement for an
operculum.  Conchiolin is a tough, semi-rigid, wear-resistant
material, and snails have developed the ability to produce that
substance.  It happens to be a brown substance.  Period.  Sometimes
we just look too hard, or take too much for granted before we start
looking.
 
Paul M.