[jamseason]

The Japanese specification for the color of traffic lights is certainly written in Japanese. If the specification says the light should have color "ao", then either blue or green or something in between that matches it is OK.

That the color region that word encompasses doesn't match that of the English words "green" doesn't really matter, and is not particularly mysterious.

[wlonkly]

The article talks about the specification and it's more... specific than that:

> “In 1973, the government mandated through a cabinet order that traffic lights use the bluest shade of green possible—still technically green, but noticeably blue enough to justifiably continue using the ao nomenclature,” Allan Richarz writes for Atlas Obscura.

[MichaelZuo]

So there was an intentional decision to adjust the lights to their conceptual preferences. Assuming someone can source this 1973 cabinet order.