There is some discussion of shell dyes in Gary Rosenberg's Encyclopedia of Seashells.  I believe the final chemical is essentially identical to indigo, also obtainable from plants, both the green photosynthesizing kind and the chemical manufacturing kind.  However, small amounts of purple dye are easily obtained by anyone with access to live dye-producing muricoideans.  Simply pick up a live murex and poke it a bit.  As you continue to collect in the sunshine, permanent purple  spots will become visible on your field clothes.  It worked for my brother.

    Dr. David Campbell
    Old Seashells
    46860 Hilton Dr #1113
    Lexington Park MD 20653 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droigate Spa