Which pigment made from crushed sea snails clothed Roman emperors?

Tyrian purple, also known as royal purple, imperial purple, imperial dye, or simply tyrian, is a reddish-purple natural dye. It is secreted by several species of predatory sea snails in the family Muricidae, rock snails originally known by the name Murex. The coloured compound is 6,6'-dibromoindigo.