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?
(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.
We were told in school it signified fire or the blood shed by the comrades, but the true story is that red color came from the Second Paris Commune (after which Communism was named), then in turn from the Great French Revolution (which started from the first Paris Commune), and ultimately from the Commune of Rome of 1144. The Commune of Rome of 1144 choose red color because it was the color of Republican Rome, the restoration of which the proponents advocated against the Papacy.
Richard Wurmbrant, in his books, stated that red ties worn by those who gave allegiance to the communist/socialist governments were the way to recognize the people who could have jobs, get necessities, etc. Those without red ties were outcasts and subject to arrests and no place to legally live, at least not for very long.