很少有人完全看不到色彩，色盲中较为常见的是人们难以区分特定的颜色，比如红绿和黄蓝。

It's quite rare for someone not to see color at all. It's more common for color blind individuals to have difficulty differentiating between certain colors, like red and green, or blue and yellow.

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You don't see a red and yellow, you don't see green and blue.

你们不可能看到红连接黄 或者绿连接蓝

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Now in a stoplight, it only has three, one is tempted to say, variables plainly differing from each other: red, yellow and green we have two ways of thinking about the red light.

交通灯，三种灯，红黄绿，它们的颜色和彼此不能,我们有两种方式认识红灯的意思。

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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