They're colored different here to indicate one new difference that you know about now, and that's that there is an orientation, there's an up and down on the chain.

这里被涂成不同的颜色,来表示另一个不同之处,它是有方向的,表示双链一条向上,一条向下

耶鲁公开课 - 生物医学工程探索课程节选