You really do learn something new every day... I found an image of a man with a purple splashed shirt and a skien of yarn on the beach. Naturally, seeing three of my favorite things (dye, yarn, ocean) in the same image got me on the search. It's the cultivation of a beautiful purple dye that comes from the Purpura shell of the Oaxacan coast. This purple shell dye, like many luxury fashion trends of ancient times, has a few wild stories behind it. It was also cultivated in Essaouira, Morrocco from the Murex shell (still not sure if Pupura and Murex create the same dye or not, but they seem to create the same color), and is currently still created there by a few artisans. All images above from here, showing the Mixtec of Oaxaca artisans process.
"The hue became wildly popular among royal celebrities of the day; Cleopatra loved the stuff so much that she dyed the sails of her royal barge purple to meet Mark Antony. But violet soon turned to violence. Legend has it that Juba’s son Ptolemy was murdered by Emperor Caligula for having the audacity to sport a purple robe, making trendy Ptolemy possibly the world’s first fashion victim. The bright, nonfading dye was never successfully produced commercially, and the secret extraction methods were assumed lost in the siege of Constantinople in 1453. But in Essaouira the stuff is mysteriously still available, for a price. The mysteries of the color purple are still passed down from one generation of murex collectors to the next, and jealously guarded." from here.