These colours are chosen because they can be seen from afar in day and night. Red is a colour of caution and gives a warning feeling to people. Green on the other hand, has a soothing effect, signalling okay to go. When the first traffic light came about, there were only 2 colours - red and green. Amber came about as a transition between the 2 colours. It had to be a colour that is easy to differentiate between red and green. white was a first choice but there were worries that there might be confusion with the street lamps. Thus, the next colour is amber.