The gray coloration in this hat is made using a dye created from the ash spewed forth from Mount Vesuvius recovered by archaeologists.*
Mount Vesuvius is a stratovolcano located on the western side of Italy, near Naples. It rises to an altitude of 1,281 meters above sea level; although the height of the volcano changes after each eruption. Vesuvius is best known for its devastating eruption in AD 79, which destroyed the towns of Pompeii and Herculaneum, burying the region under meters of ash, and killing an estimated 10,000 - 25,000 people.
*Everything after this point is true (according to our history class), the stuff before... not so much.