素问·经络论 Treatise on the Channels and Connecting Vessels
黄帝问曰：夫络脉之见也，其五色各异，青黄赤白黑不同，其故何也。歧伯对曰：经有常色，而络无常变也。帝曰：经之常色何如。歧伯曰：心赤，肺白，肝青，脾黄，肾黑，皆亦应其经脉之色也。帝曰：络之阴阳，亦应其经乎。歧伯曰：阴络之色应其经，阳络之色变无常，随四时而行也。寒多则凝泣，凝泣则青黑，热多则淖泽，淖泽则黄赤，此皆常色，谓之无病。五色具见者，谓之寒热。帝曰：善。
Huangdi asked: When the connecting vessels become visible, their five colors differ — green, yellow, red, white, and black are not the same. What is the reason for this? Qibo replied: The channels have constant colors, while the connecting vessels change without constancy. Huangdi said: What are the constant colors of the channels? Qibo said: The heart is red, the lung white, the liver green, the spleen yellow, and the kidney black — each corresponds to the color of its own channel vessel. Huangdi said: Do the yin and yang aspects of the connecting vessels likewise correspond to their channels? Qibo said: The color of the yin connecting vessels corresponds to their channel, but the color of the yang connecting vessels changes without constancy, following the movement of the four seasons. When cold predominates, the blood congeals and stagnates, and this congealing and stagnation produces green-black; when heat predominates, the blood becomes moist and flowing, and this produces yellow-red. These are all normal colors, said to indicate the absence of disease. When all five colors appear together, this is said to indicate cold and heat. Huangdi said: Good.