沈砺《咏史》；“渭浊泾清本异流，当涂符谶趁时收。”

泾渭分明

The difference is as distinct as that between night and day.

成语解释：泾水清，渭水浊。用以比喻事物和人品的差别。
常用程度：一般
感情色彩：褒义词
语法用法：作宾语、定语；用于比喻句
成语结构：联合式
产生年代：现代