In Japan, the color green is often referred to as “ao,” which historically meant blue. This is because the Japanese language originally had only four basic color words: black, white, red, and blue. When the word “midori,” meaning green, entered the language, it was still considered a shade of blue. This linguistic challenge has had lasting effects on Japanese culture, including the labeling of green objects as blue. In the case of traffic lights, Japan compromised by using a bluish shade of green that technically satisfies international regulations but can still be referred to as “ao.” It’s a bureaucratic solution to a linguistic quirk.
https://www.rd.com/article/heres-japan-blue-traffic-lights/