there is lots of room for more - and more in-depth - information, but meanwhile you'll find an assortment of answers in these threads

• Why is Communism "red"? - /u/intangible-tangerine kicks things off with Roman slaves; many other responses on other reasons from modern countries (France, Russia, China)

• /u/facepoundr , /u/DonaldFDraper and /u/ainrialai all talk about the 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?

• When and how did the solid red flag become a symbol for socialist/communist/egalitarian uprisings?

• How did red become the color of Communism?

• Why was Red such a predominant colour in Communist Russia and Chinese Propaganda? - /u/blindingpain adds more layers of meaning for Russia

if you have follow-up questions on archived posts, ask them here & mention the relevant user's username to notify them