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?
(There's a pretty complete description of the association between the red flag and revolution on Wikipedia's page on the color red)
It can be associated with revolution (at least as far back as the French revolution, before that in Europe it was sometimes associated with monarchy) and communists see themselves as revolutionaries.
Red is associated with courage, sacrifice, blood and war in general.
In the West, a red flag in battle was used to signal the fight would be to the death and there would be no surrender (and/or no prisoners taken). Communists may have adopted it to send that message to the nobles/upper-classes. Or maybe it was just for the general association with courage/sacrifice/war.
In Asia, and China in particular, red is associated with loyalty, honor, success, and happiness. As such it's a nice flag for the Communist Party to use.
I don't think there's a definitive logical answer to the question. The answer is probably some combination of the above. Also communists world-wide would have copied each others color, to symbolize unity/shared ideology and the historical association with revolution.
In Europe and especially in France the events of the Paris Commune and the use of the red flag there set for good the red flag as the symbol of the socialist revolution and not "just" the revolution.