Final answer:
The purple dye derived from shellfish that became a favorite color of royalty is known as "murex" and was used in the highest echelons of Byzantine society.
Step-by-step explanation:
The purple dye derived from shellfish that became a favorite color of royalty is known as "murex". This dye was used in Byzantine society, specifically among the highest echelons of society. It was an extremely expensive dye and was often used to dye silk textiles, giving them a rich purple hue.