An old nickname for blue.

In ancient times, blue was called indigo, azurol, azurol and indigo. The application of indigo pigment in fabric dyes can be traced back to 2500 BC. Azure and indigo are traditional color nouns in China, which are deep and clear blue-green and dark blue respectively. Blue is the color that describes the clear sky, which is recorded in Tang Du Fu's poem "A View of Jin Huashan in Winter".

An old nickname for blue.

Blue is a high-frequency light with a frequency of 600-700 Hz in the visible light of electromagnetic waves (second only to violet light). Blue is one of the three primary colors and four psychological colors of light, which is similar to the color of clear sky.

The contrasting colors of blue are orange and yellow, and the adjacent colors are green and purple. Blue is a symbol of eternity. There are many kinds, each of which represents a different meaning.