Indigo is extracted from indigo plants, but it is bluer than the plants from which it comes.
Pinyin Q and ng chúlánérèsèng yúlán
Explain cyan: indigo, cyan pigment. Blue: Polygonum cuspidatum, a kind of grass that can extract pigment. Indigo is extracted from Polygonum cuspidatum, but its color is darker than Polygonum cuspidatum. Metaphor means that people can be improved through study or education. It is often used to indicate that students surpass teachers or that future generations surpass predecessors.
The source of Xunzi's Persuasion: Qing, which is taken from blue, is blue.