Why is the color scheme of the city hall red and green?

Respect red. Red and green are a pair of complementary colors, that is to say, when red and green are together, it will cause great visual contrast and strongly stimulate people's senses, which will make people feel that red is redder in this strong background, and strong red is what we are pursuing. There is only a small area of green under the Great Hall of the People, but it will make us pay more attention to a large area of red.