The Romans divided the combatants in the circus into classes, called factions, each class being distinguished by its special colour, like the crews of a boat-race. The four original factions were the leek-green (prasina), the sea-blue (veneta), the white (alba), and the rose-red (rosea). Two other factions were added by Domitian, the colours being golden-yellow (aurata ) and purple. As these combatants strove against each other, and entertained a strong esprit de corps, the word was easily applied to political partisans. In the faction riots of Constantinople, A.D. 532, above 30,000 persons were killed. (Latin, factio).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Faction from Fact Monster: