Red color is traditionally associated with communism. You can find it at Soviet Union flag, China flag and red star is normally symbol of communist countries  . Why is red color associated with communism? What does it symbolizes?

The color red is traditionally associated with communism. You can find it in the flags of the Soviet Union and China, and the Red Star is a a common symbol in communist countries. Why is the color red associated with communism? What does it symbolize?