E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.)