《汉书·志·律历志上》
地的中间数字是六，六作为律音，律有形有色，颜色崇尚黄色，五色中再也没有比它更艳丽的了。
地之中数六，六为律，律有形有色，色上黄，五色莫盛焉。
The middle digit of the earth is six. Six is the musical note. The musical note has shape and color. The color is yellow. Among the five colors, there is no more gorgeous one.