[fenomas]

I just scanned a few articles on this. TFA and its source don't mention this, but the first JP traffic law for traffic lights (from 1930) used midori/green, following US standards. But apparently newspapers mostly used "ao" for whatever reason, and that was the usage that became widespread. Then later on (1947) the traffic law was updated to say "ao", presumably to match common usage. Of course the actual lights were green the whole time.

Then in the 1970s the rules changed to recommend bluer shades of green, and new lights made afterwards reflected that. None of the articles I found gave a concrete reason; a few suggested bluer lights were easier to see, and a few implied the change was motivated by the "ao" name.