Indigo Blue is Extracted From The Indigo Plant

During the Northern Dynasties Period, there lived a man named Li Mi, who from his youth studied under Kung Fan. Kung Fan was both very knowledgeable and morally upright. He taught his students to be conscientious and responsible. Before each lecture, he would first thoroughly research his subject, and then teach his students everything that he knew. He had very high hopes for his students, especially in the area of morality, and controlled them very rigidly. So his students all had high respect for him. Li Mi was an extremely intelligent and hardworking student. He not only memoized everything the teacher taught him, but because he studied very intensively, he learned many new things. Several years late, he knew even more than his teacher.

From then on, whenever Kung Fan had a problem, he would go ask Li Mi for advice. This made Li Mi very uncomfortable, so his teacher used a phrase of Shum-tz "Indigo blue is extracted from the indigo plant" - as a metaphor for a student surpassing his teacher. (This is because the color, indigo blue, is a more beautiful shade of blue than the plant from which it is extracted.)

The meaning of this idiom, then, is that "the student has become better than the teacher."

青出於藍

子璠是北朝一位著名的老師，學識品德都很好，教學既認真又負責；他門下有一個學生李謐，不但聰明又肯用功，老師教的課業，他不但一一加以熟讀，並且能夠舉一反三、融會貫通成為自己的思想。時間一久，李謐有許多精闢透徹的見解都超過了老師。

有一次孔璠向李謐請敎一個問題，李謐心想老師怎麼反過來向我請教呢？因此惶恐得不知如何是好。孔璠很誠懇的對他說：『李謐，不必這樣，聖人都沒有固定的老師了，更何況是我們呢！只要有一技之長的，都可以當我的老師，你就不要客氣了吧！』這件事後來被傳揚出去，大家都非常感動，他們還作出一首曲子，來宣揚孔璠這對師徒呢！「青成藍，藍謝青；師何常，在明經。」意思是說，「責」這種染料是從一種叫「藍」的草本植物的葉子中提鍊出來的，顏色卻比藍更好看。李謐由於聰明和努力，以致學問高過老師，孔璠卻不忌妒，反過來向他請敎問題，由此看來，老師怎麼會只有固定的一個人呢？凡是有學問的人，都可以作自己的老師啊！其實，「青出於藍」本來是周朝的荀子說的，他說：「青從藍取出，卻比藍悅目；冰是水做的，卻比水更冷。」後來的人，拿來形容學生的學識超過老師。