This was an exercise in color symbolism. Purple often indicates royalty and wealth. Because it's (relatively) rare in nature, and purple dyes used to be difficult to make and therefore expensive, only the wealthiest could afford to dress in purple.

“Her clothing is silk and purple.…”
- Proverbs 31:22

“And Mordecai went out from the presence of the king in royal apparel of blue and white, and with a great crown of gold, and with a garment of fine linen and purple: and the city of Shushan rejoiced and was glad.”
— Esther 8:15

“There was a rich man who was clothed in purple and fine linen and who feasted sumptuously every day.”
— Luke 16:19

“They put a purple robe on him, then twisted together a crown of thorns and set it on him. And they began to call out to him, ‘Hail, king of the Jews!’”
— Mark 15:17-18