Comment by numpad0

I think this might be a bit overblown. "why do we call it blue signal?" is a common 3-5 years old question in Japan.

Old Japanese traffic signals had blue tinted lenses, like ultramarine blue. Those lenses were used in conjunction with warm yellow incandescent lamps, technology available at the time. Deep blue + warm yellow = green.

Over time the green color must have normalized, without laws and slogans not reflecting that. And nowadays they're green LEDs.