system
A chat on subreddit /r/NoStupidQuestions
human
Nature.  Red = blood = bad.  Green = trees = good.

But we got the idea more directly from train flag systems.
gpt
I vaguely recall that in the original rail configuration, red lights meant "stop", and *white* lights meant "go".

It took a little while, but they eventually realized that the red glass they put in front of the white light could occasionally break or fall out, and this had consequences, and thus they needed to come up with a different color that meant "go".  

*What color light is up ahead?*  

*White.*  

*Uhhhh....*

Enter green.