Which color could only be worn by a Roman Emperor?

Player #13640194, purple, my favourite colour, beautiful

The colour came from the seashell Murex. It was invented by the Phoenician in the city of Tyr by a secret complicated composition. Herodotus mentioned that all Tyrian were extremely rich mostly from the trade of this purple textile all over the old world, until Alexandre sadly destroyed the city!

The dye was made from a sea snail and incredibly costly thus the reason for nobility only. Not sure about the purple robe on Jesus, no really high ranking Romans in Judea at that time who would have been able to wear the purple and owning such a garment would have been akin to treasonable, let alone using it as a symbol of mockery.

that is my favorite color

Though it wasn't a true purple as we now know it. More of a burgundy.

Thank you Rick Riordan!!