The green, black, and red faces were both beautiful and charming. Which poem did it come from? What did the poem mean?

This poem comes from the thirty-first chapter of Bai Juyi's Song of Everlasting Regret in the Tang Dynasty. It is as follows: ``` Green Dai, red Yan, two hair, thousands of charming love without rest. Natural beauty is hard to imitate, and perfection is impossible to paint. The king covered his face and could not save himself from seeing the blood and tears flowing in harmony. The yellow dust is scattered, the wind is bleak, the cloud stack is winding, climbing the Sword Pavilion. At the foot of Mount Emei, there are few people walking. The flag is dark and the sun is thin. The water of Shu River is green, and the Lord Green of Shu Mountain is in love day and night. I see the moon in the palace, sad face, night rain, hear the sound of the bell, heartbroken. The world spun back and forth, supporting the strength of the dragon and tiger dance in the air. The king covered his face and could not save himself from seeing the blood and tears flowing together. The yellow dust is scattered, the wind is bleak, the cloud stack is winding, climbing the Sword Pavilion. At the foot of Mount Emei, there are few people walking. The flag is dark and the sun is thin. The water of Shu River is green, and the Lord Green of Shu Mountain is in love day and night. I see the moon in the palace, sad face, night rain, hear the sound of the bell, heartbroken. ``` This poem described the love story between Emperor Xuanzong of the Tang Dynasty and Yang Guifei."Lvdai Hongyan" referred to Yang Guifei's beauty, while "Qianjiao Baimei" described her charm. Through the love story between Emperor Xuanzong of the Tang Dynasty and Yang Guifei, the whole poem shows the beauty and cruelty of love and the complexity of human nature.