Where did the name Qing Dan in The Book of Songs come from?

I don't remember a clear word in the Book of Songs.

But there are Dan's Book of Songs, National Style, Qin Feng and Zhongnan.

Original text:

What's in Zhongnanshan? There are mountains and plum trees. A gentleman came to this place, dressed in fine clothes and fur. Blushing like Tudan, is he my Lord?

What's in Zhongnanshan? It is spacious and angular. A gentleman came to this place, wearing a turquoise jacket and colorful clothes. Yu Pei Yu Pei, riches and honour will never be forgotten.