In article , Peter Corser
writes
BTW - did you know that the original signal aspects on main line railways
were white for clear and red for stop or caution (distants were not well
differentiated in the early days). I do not know when the green aspect
became the standard, but suspect that it was in the early years of the
twentieth century as electric lighting external to the railway became
common.

Green used to be the caution aspect:

White is right and red is wrong.
Green means gently go along.

My memory says it was around WW1 that the caution aspect moved to
yellow, allowing green to mean clear.

On the big railway, shunters' handlamps show white for go and green for
slow down.

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