Good question! Traffic light is called "青信号(あおしんごう：Blue light)" in general. Why it is called blue though the color itself is green? Long long time ago, green was called blue in Japan, because making beautiful green-dye was too difficult and green was never treated as basic color. Speaking of the basic color, it is white, black, red and blue. That's why these colors are still treated as special color even for grammar. Only these colors could be i-adjective as it is; あかい(red), あおい(Blue), くろい(Black), しろい(White). Except the traffic signal, green apple（あおりんご）, green chili pepper（あおとうがらし）, leafy green vegetable(あおな) are representative for describing as "blue." Hope it somehow helps you😊