It's the color of blood!!!!

"It has been associated with left-wing politics since the French Revolution. The banner of the Paris Commune of 1871 was red and it was at this time that the red flag became a symbol of communism". (Seriously, wikipedia takes 1 second).http://en.wikipedia.org/wiki/Red_flag It's all in there.

from Wikipedia: I looked up the color red and Communism The five-pointed red star, a pentagram without the inner pentagon, is a symbol of communism as well as broader socialism in general. It is sometimes understood to represent the five fingers of the worker's hand, as well as the five continents. A lesser known suggestion is that the five points on the star were intended to represent the five social groups that would lead Russia to communism: the youth, the military, the industrial labourers, the agricultural workers or peasantry, and the intelligentsia. Karl Marx and Friedrich Engels used the red star as a symbol[citation needed]. It was also one of the emblems, symbols, and signals representing the Soviet Union under the rule and guidance of the Communist Party, along with the hammer and sickle. The star has since become a symbol representing socialism of all varieties. The five pointed star is an ancient symbol used in the mystical traditions of Middle Eastern religions (Judaism and Christianity) to represent the heretical idea that the sacred inheres in humanity[citation needed]. It was for this reason that Marx and Engels, as radical humanists, were attracted to the symbol. They made it red to signify the blood lost in struggle, and to show that all humans have the same blood (ancestry) regardless of race, gender, or class. However its origins in a mass political movement are found in the Russian civil war and the end of the First World War, though the creator (if any) is unknown. It is most often thought that Russian troops fleeing from the Austrian and German fronts found themselves in Moscow in 1917 and mixed with the local Moscow garrison. To distinguish the Moscow troops from the influx of retreating Russians the officers gave out tin stars to the Moscow garrison soldiers, to wear on their hats. When those troops joined the Red Army and the Bolsheviks they painted their tin stars red, the color of socialism, thus creating the original red star.[1] Another claimed origin for the red star relates to an alleged encounter between Leon Trotsky and Nikolai Krylenko. Krylenko, an Esperantist, was wearing a green star lapel badge; Trotsky enquired as to its meaning and received an explanation that each arm of the star represented one of the five traditional continents. On hearing this, he specified that a similar red star should be worn by soldiers of the Red Army.

12-31-2016 Red is short for radical, in Russian and in English.