Why are street signs divided into blue and green?

Why are street signs divided into blue and green? There are national standards for the color of street signs. In the "Urban and Rural Geographical Names Signs", it is stipulated that: in the east-west direction, including the streets (lanes) diagonally from east to west, the background color of street signs is blue, and the font color is white; North-South Diagonal Street (Lane), including North-South Diagonal Street, with green street signs and white fonts. The following is the introduction of traffic lights: 1, the function of traffic lights: traffic lights are traffic lights that direct traffic operations, generally consisting of red lights, green lights and yellow lights. Red light means no traffic, green light means no traffic, and yellow light means warning. 2. Classification of traffic lights: Traffic lights are divided into motor vehicle lights, non-motor vehicle lights, crosswalk lights, direction lights (arrow lights), lane lights, flashing warning lights and road and railway intersection lights. @20 19