原: 論語, 八佾/8
子夏問曰：「『巧笑倩兮，美目盼兮，素以為絢兮。』何謂也？」子曰：「繪事後素。」曰：「禮後乎？」子曰：「起予者商也！始可與言詩已矣。」

繪事後素(회사후소,huìshìhòusù,かいじこうそ), painting work after white
Horse before carriage. Essentials or canvas come first before formalities or decorations.

This is from a passage where Gong-ja reminds his disciples that a person ought to develop his character first before adding on other stylistic things (文飾).

Jaha (字: 子夏, 卜商) asked, “What does this text mean? ‘Dimples from a beautiful smile are pretty, pupils from beautiful eyes are clear. It is like painting with white silk (素以爲絢).'” Gong-ja (孔子) replied, “You paint after you prepare a white canvas (繪事後素).” To which, Jaha asked again, “So etiquettes/formalities (禮) come after?” And at this, Gong-ja expressed his great pleasure with Jaha’s reply and said that he is now ready to discuss poetry (詩).

Jaha realized that formalities (depicted as a painting (絢)) are expressions of the heart (depicted as a white canvas (素)), and the principle of poetry is the same. This is why Gong-ja praised his reply, and considered him ready to discuss poetry.

The mainstream interpretation of this was reinforced by Zhu-ja (朱子, 朱熹, 1130-1200) of Song (宋) dynasty, but a different interpretation was given earlier during later Han (漢) dynasty by Jung Hyeon (鄭玄, 127-200) who read this as painting white (素) after painting with colors (繪), so as to filling in the spaces between colors with white.