We had a thread on shell dyes before. Didn't someone say that the dye of
the indigo plant is chemically the same as that of the murex? Indigo is no
longer grown much because the dye can be synthesized from coal tar. The
blue dye used for blue jeans is the same or a very similar compound, is it
not? What makes it purple in one case and blue in another?

Andrew K. Rindsberg
Geological Survey of Alabama