Some are yellow, some are red, and some are green - does anybody know what this is?

一些是黄色,一些是红色,一些是绿色的,有人知道这是什么吗

耶鲁公开课 - 生物医学工程探索课程节选

One is that there's four different colors and so you can see red, blue, yellow, green here - four different colors and that's all there are, there aren't more than four.

一是这些梯阶有四种不同颜色,你可以看到红 蓝 黄 绿,恰恰只有四种不同颜色,不多不少

耶鲁公开课 - 生物医学工程探索课程节选