He wrote in an essay that he could see early on that Yen was steady and reliable, able to anticipate opportunities but not moving to action until he had a firm plan. Koo could tell that he would definitely make a success of himself.
他曾寫過一篇感想錄表示，早年與顏雲年初識時，就看出他穩健踏實，謀定而後動，且能洞燭機先，將來必有成就。