Acutally, you are just imagining the colors.  They are pigments of your imagination.

Seriously, investigating them can be difficult.  You have to find a way to extract enough of the pigment to be able to analyze it.  This often requires destroying the shell, and may require large numbers of shells to generate enough material.  Chemically separating out the component may also be difficult.

The color of true nacre is due to structural patterns reflecting light rather than pigmentation.  I do not know of other examples in mollusks, but many natural colors are produced this way.  Try getting a blue feather (not painted but from a blue colored bird) and holding it up to a light.  It's gray, but the structure of the feather reflects blue light.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!