Why are street signs divided into blue and green? What is the reason?

The reason why street signs are divided into blue and green is to distinguish ordinary roads from expressways. Blue signs of ordinary roads; Green signs of expressways and expressways. In addition, east-west street signs, including east-west oblique streets (lanes), have blue background and white font color; North-south trend, including north-south diagonal streets (lanes), street signs with green background and white font color.