「赤信号と青信号」－Red and 'green' traffic light

Red traffic light：赤「あか」meaning red ＋信号「しんごう」meaning traffic light

'Green' traffic light：青「あお」meaning blue＋信号

Japanese used to only distinguish the major colours as black, white, red, and blue, and green (緑「みどり」）was not commonly used to describe the colour of things. Instead '緑′ originally represented freshness, especially of forests or nature, and only later developed to signify the colour green as well. Therefore, when the first traffic light was invented, the green light was commonly referred to as blue, and this label has continued today. You can sometimes see traffic lights in Japan with a more obviously blue colour due to there not being a distinction in their naming.