How old is the convention that red means warning or stop, while green means OK? And what is its origin? If it was an arbitrary choice, then it would seem to be a strange one since difficulty in distinguishing red and green is the commonest form of colour-blindness.

At least since 1860 when the first “red for stop, green for go” railway lights were in use (though for all I know it might be a lot older)

If it was an arbitrary choice, then it would seem to be a strange one since difficulty in distinguishing red and green is the commonest form of colour-blindness.

Even red/green colorblindness is rare and it’s not the sort of thing that people talk much about, so I would be very surprised if it would even occur to anyone who is not colorblind that they should consider it when picking the colors for signal lights. I suspect a more important consideration was simple visibility. Yellow could be too easily mistake for an “ordinary” light and blue is too hard to see from a distance.

Red lightwaves are longer than blue and penetrate interference (e.g. air and water molecules, suspended particles, etc.) better. Green lightwaves are somewhere in between. At least that’s what I was told long ago. Don’t ask me why the sky is blue mid day.

Surprisingly, human eyes have red, green, and *blue receptors only. We see red, green, and blue rather well. All the other colors that we perceive are mixtures of these receptors being stimulated and sending signals to the brain. To make yellow on a TV screen or computer monitor, for example, you mix red and green lightwaves. I suspect that the reason yellow gets lost in the distant background more easily than other colors is because it is closer to pure white light.

*edit: Actually it isn’t even blue, but some shorter frequency to the left of blue. So-called violet? Anyway, this may be another reason why blue is not as intensively visible as red.

edit2: The best way to understand the fact that color is a construct of perception (that it is a partial illusion) is to see that a black and white photgraph of a rainbow does not show the bands of color we so readily associate with the phenomenon. The reason for the bands lies in physiology, not physics.

The first citation for green light in the OED is:

1839 Roads & Railroads, Vehicles, & Modes of Travelling xviii. 330 “A green light should be placed at each station at the spot where the engine-man should slacken his speed, and a red light at the point where he is to stop.”

So the association of red with stop is at least that old.