Legend has it that before Lao Tzu crossed Hangu Pass, Guan Yinxi saw purple gas coming from the east and knew that saints would pass by. Sure enough, I came riding a green cow. A metaphor for good luck. Didn't the emperor call the palace the Forbidden City? That's what he meant.
In the Qing dynasty
Before the Qing dynasty entered the customs, it established political power and built a palace in Shenyang (now the Forbidden City in Shenyang). After entering the customs, because it originated in the northeast, I made an "eastward tour" from time to time to commemorate my ancestors. "Purple Gas Coming to the East" was written during the Qianlong Crusade, and now it is hanging in the Forbidden City in Shenyang, which probably means this.