How has "素以为绚" be rendered into English?
You asked a hard question. 

The original text of Analects is as following.

子夏曰、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。


There is a wide range of opinions in the interpretation of 素以為絢.

子夏 was a discipline of Confucius. He asked Confucius the meaning of an ode 碩人 in "The Book of Song" (詩經). The ode praises a noble lady 莊姜, who was the wife of the Duke of Dukedom 衞.

Unfortunately, the known version of 碩人 contains only 巧笑倩兮 and 美目盼兮, but 素以為絢兮 is missing. This breeds speculations and opinions from Han Dynasty to nowadays.

Confucius did not explain 巧笑倩兮 and 美目盼兮. They are too obvious. He explained 素以為絢 with 繪事後素.

James Legge translated 素以為絢 as "The plain ground for the colors" and 繪事後素 as "The business of laying on the colors follows (the preparation of) the plain ground". This is what the majority of scholars interpret it. 


  For simplicity, before you draw or paint, you need a plain ground. On
  a plain ground you could draw and paint anything colourful.
  This is what 素以為絢 means.


The story does not end here. After the explanation of Confucius, 子夏 connected it with 禮, namely ceremonies, which is the core theme in Confucius study. (If a person with qualities of plain ground, namely, honest and trustful, he would learn 禮 well.)

Confucius commented that 子夏 got what his mean and then he had the qualification to discuss the odes in "The Book of Song".

The odes are full of allusions and metaphors. They requires a learner not only understanding the literal meaning of the words, but also getting the underlying meaning of odes and connecting them to life.

The story of 素以為絢兮 in the Analects illustrates how to learn and use "The Book of Song".  It is more than the words 素以為絢.