Give the Green Light

Rooted in real railway signalling. Coloured lights were adopted by railroads in the 19th century to tell engineers whether it was safe to proceed, and green came to mean go. Interestingly, green wasn't the original "go" colour on railways, white was used first, before safety concerns (a broken red lens could be mistaken for white) led to the colour scheme being revised. Road traffic signals adopted the same red-amber-green logic in the early 20th century, standardised further once agencies began regulating traffic lights, and the phrase moved into general figurative use for granting permission in the first half of the 1900s.