hi! not discouraging further discussion here, but a quick search turns up two answers:

French Revolution

• When and how did red become the 'official colour' of Communism? Were/are there any Communist movements that didn't/don't associate themselves with the colour red?

• Why are Communists 'Reds'?

Symbolism in Russian culture

• Why was Red such a predominant colour in Communist Russia and Chinese Propaganda?

• Why is Communism "red"? - also tracks back to Roman slave revolts

a thread touching on both

• How did red become the color of Communism?

if you have followup questions on locked posts, ask them here & include the user's username so they'll be auto-notified

By the time of the Russian Civil War, the color red had come to represent revolution (the French Revolution really popularized this). Lenin's Bolsheviks claimed the color as their own early in their fight against the rural "white russian" Mensheviks. The large, urbanized and well supplied Bolshevik fighting force became known as the Red Army, and red just happened to be a popular color to suggest death and danger for western journalists (Nazi Germany's colors used a lot of red). Given this overlap, and the color of the new Soviet (and Chinese) flag, red stuck as the universal color of Communism.