During the Spring and Autumn period, there was a horse appraiser in the state of Qin named Sun Yang. Every horse he selected was a steed capable of covering a thousand li by day and eight hundred li by night. His skill became legendary, and since mythology spoke of a celestial star named Bole who oversaw the heavenly horses, people began calling Sun Yang by that name as well.
One day, Duke Mu of Qin asked him, "You are old now. Who can take your place in judging horses for me?" Bole replied, "There is a man named Jiu Fanggao, whose ability to judge horses is no less than mine. I can invite him to meet you."
When Bole recommended Jiufang Gao to Duke Mu, the duke asked him to find a fine horse
Three months later, Jiu Fanggao returned and said he had found an excellent horse. Duke Mu asked what it was like, and he replied, "A yellow mare." The duke sent someone to check, and the person reported back, "A black stallion."
Duke Mu summoned Bole and said, "The horse expert Jiu Fanggao you recommended found me a horse that is clearly male, but he said it was female; clearly black, but he said it was yellow. I truly doubt whether he knows how to judge horses!"
Bo Le went out to examine the horse, then returned in surprise and delight, saying, "Jiu Fanggao is indeed more skilled than I am—that horse is truly a rare and excellent steed." Duke Mu was very displeased and said, "He can't even tell the horse's gender or coat color, yet you praise him!"
Bo Le gave a thumbs-up and exclaimed, "Your Majesty, when Jiu Fanggao judges a horse, he focuses not on its appearance but on its inner essence—he sees what he needs to see and ignores what he doesn't. This method of observing things holds far greater significance than just judging horses!"
Duke Mu ordered the horse brought forth, and upon seeing it, he exclaimed, 'This is indeed a rare thousand-li steed, unmatched in all the land!'
Later, the idiom "A Black Mare or a Yellow Mare" came to be used as a metaphor for the superficial appearance of things.
Source: *Liezi*, Chapter "Explaining Signs"
Meaning of the Idiom: Later, the Chinese idiom "牝牡骊黄" came to describe a metaphor for the superficial appearance of things.