"当颜色变绿"用英语表达为 "when the color turns green"。
其中, "turns" 是动词, 意思是 "变得, 变成"。 "green" 是形容词, 意思是 "绿色的"。
下面是一些例句：
The traffic pght turns green, you can go now. (交通灯变绿了, 你可以走了)
When the leaves turn green, it's a sign of spring. (当树叶变绿时, 这是春天的标志)
The grass turns green after a long period of drought. (在长期干旱之后, 草地变绿了)