So I just learned this today because an Anki card fucked me up and I decided to google why the hell the word for "blue" was being used to say that a traffic light turned green.
>The Japanese word ao (青, n., aoi (青い, adj.)), the same kanji character as the Chinese qīng, can refer to either blue or green depending on the situation. Modern Japanese has a word for green (緑, midori), but it is a relatively recent usage.[citation needed] Ancient Japanese did not have this distinction: the word midori came into use only in the Heian period, and at that time (and for a long time thereafter) midori was still considered a shade of ao. Educational materials distinguishing green and blue came into use only after World War II; thus, even though most Japanese consider them to be green, the word ao is still used to describe certain vegetables, apples, and vegetation. Ao is also the word used to refer to the color on a traffic light that signals one to "go". However, most other objects—a green car, a green sweater, and so forth—will generally be called midori. Japanese people also sometimes use the word gurīn (グリーン), based on the English word "green", for colors. The language also has several other words meaning specific shades of green and blue.
