Why are the colours red, green and amber chosen for traffic lights?
In the daylight, you can see the colour red more clearly than any other colour. It has the longest wavelength: 740 – 620 nm. That means red is visible from the farthest distance. It stimulates the nerves, being a warm colour. It is also the colour of blood and spells danger, hence “STOP”.
Green is also clearly visible, having a medium wavelength of 575 – 500 nm. It is a cool colour, the colour of grass, soothing to the nerves. It signifies peace and safety and hence “GO”.
Amber is another colour clearly visible, having a wavelength of 620 – 585 nm, between red and green in the visible light section of the electromagnetic spectrum. Hence, an amber light is used as a warning between “STOP” and “GO”.