Derogatory slang for a
Communist, especially of the
Soviet or
Chinese persuasion.
Lenin and
Mao both used the
colour red as their
symbol.
In the Bolshevik revolution and the Russian Civil War, the Red Army was that of the Bolsheviks (as opposed to the Whites, who were tsarists; and the Mensheviks, who AFAIK didn't have a colour).