What does it mean to take off your purple clothes and change into a red robe in fortune telling?

Hello, as far as I know, there is only one saying that ordinary scholars in ancient times wore blue shirts, and "red robe" refers to official clothes. This is a saying:

The noblest flesh and blood in life entered the name label of Hong Men early; It's thirty-six years old, so I'm going to take off my blue shirt and change it into a red one.

If it is really a purple robe, it existed in ancient times, and only senior officials with more than two products wear purple robes. If the purple robe is changed into a red robe, it means demotion.