What do you mean by "purple gas coming from the east"

Purple gas comes from the east, z ǐ qdng lá i

Meaning-Legend has it that before Lao Tzu crossed Hangu Pass, Guanling let Yin see the purple gas coming from the east and knew that there were saints to pass. Sure enough, I came riding a green cow. This old metaphor is a symbol of good luck.

Source-Historical Records Biography of Lao Fei.

Extended data allusions-

Laozi is knowledgeable and served as an official in charge of books and classics in the Zhou Dynasty. About the time he was in his seventies, there was chaos in the world, and the princes fought for power.

"Purple gas comes from the east" is a great victory in "Laozi Going through the Customs", which has been passed down through the ages.

Discrimination of idioms-

Purple gas is purple gas, and the ancients thought that purple was related to being an official. The uniforms of many officials are purple. Purple gas is mainly used for the plaque of scholarly family.

References:

Purple gas from the East-Baidu Encyclopedia