Hello Leslie, Oliver and others.
For those interested in shell patterns, I recommend the booklet "Remarks on
colour paterns and related features of the Molluscan shells" by J.J.
Oberling. It has 56 pages of text and 11 B&W plates to explain how various
patterns are created.
I do not believe it contains any details regarding chemical makeup of the
color pigments, but it is a fascinating study of shell patterns.
Winston Barney