Gastropods which live in an environment with high iron content often
show a reddish brown coloration in the shell, due presumably to
oxidized iron (fancy name for "rust") incorporated into the shell.  I
have not had the experience of collecting Cypraea from shipwrecks.
However, there is an interesting example of this phenomenon in the
Cape Cod Canal (the world's widest manmade canal, on Cape Cod,
Massachusetts, U.S.A.).  Both shores of this canal are lined with
large rocks.  Two large steel suspension bridges cross the canal from
the mainland to the Cape.  Specimens of Nucella lamellosa (Atlantic
dogwinkle) in the canal are typically white in color (the more
colorful forms of this species are primarily more northern in
distribution).  However, specimens collected on the concrete footings
of the two suspension bridges show a red-brown coloration, presumably
from dissolved iron washing down from the bridges.  A hundred feet
away from the bridge in either direction the specimens are white.
What is not clear in such situations is whether the snails actively
extract iron from the water and deposit it in their shells along with
the calcium, through their mantles - or - whether iron replaces
calcium in the shell from without, by a simple inorganic reaction,
without metabolic involvement of the snail.
Regards,
Paul M.
Rhode Island, U.S.A.
"The only island not surrounded by water"