What does it mean that the river is as green as blue in spring?

It means that the green river is greener than the blue grass.

It's from the Tang Dynasty poet Bai Juyi's Memory of Jiangnan.

Jiangnan is good, and the scenery has always been familiar.

at sunrise, the river is more red than fire, and in spring, the river is as green as blue. How can we call people not miss the south of the Yangtze river?

Translation:

How beautiful the scenery in the south of the Yangtze River is, and the scenery has been familiar for a long time. The sun rises from the river, making the flowers on the river brighter than red, and the green river is greener than the blue grass. How can people not miss Jiangnan?

Bai Juyi once served as the secretariat of Hangzhou, staying in Hangzhou for two years, and later served as the secretariat of Suzhou for more than a year. In his youth, he roamed the south of the Yangtze River and lived in Suzhou and Hangzhou. It should be said that he had a good understanding of the south of the Yangtze River, so he was deeply impressed by it. Twelve years after he retired from Suzhou Secretariat due to illness and returned to Luoyang, at the age of 67, he wrote three poems recalling Jiangnan, which shows that the scenic spots in Jiangnan are still vivid in his heart.