Chinese Idiom Stories
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."