Mosaic of a blue charioteer, originally from the Villa dei Severi at Baccano (16 miles from the Via Appia); now in the Palazzo Massimo alle Terme. Photo source.
What made chariot racing distinctive was the power and popularity of the factions. There were four factions in chariot racing (we would probably call them teams): Red, Green, Blue, and White – and the colours and factions were the same across the empire. Each had their supporters, but the Greens and Blues seem to have been the most popular. However, the first faction we hear of, in the 70s BCE, is the Reds; apparently a supporter of Felix, a charioteer for the Red faction, was so distressed that he threw himself onto the funeral pyre; others said he had just been overcome by the fumes from the incense on the fire and fell in, which in itself is an achievement of sorts:
We find it stated in the Annals, that when Felix, a charioteer of the Reds, was placed on the funeral pile, one of his admirers threw himself upon the pile; a very stupid way to behave. In case, however, that this event might not be attributed to the great excellence of the dead man in his art, and so add to his glory, the other parties all declared that he had been overpowered by the strength of the perfumes.
Pliny the Elder, Encyclopaedia 7.53
The top two factions seem to have been the Blues and the Greens. Caligula was a devoted supporter of the Greens, a devotion that extended to one of the horses in the Green stable.
Caligula was so passionately devoted to the Greens that he constantly dined and spent the night in their stable, and in one of his parties with them he gave the driver Eutychus two million sesterces in gifts. 3 He used to send his soldiers on the day before the games and order silence in the neighbourhood, to prevent the horse Incitatus from being disturbed. Besides a stall of marble, a manger of ivory, purple blankets and a collar of precious stones, he even gave this horse a house, a troop of slaves and furniture, so he could entertain the guests invited in his name more elegantly – it is also said that he planned to make him consul.
Suetonius, Caligula 55.2
(For more on imperial fans of the games see this page.)
Until the 300s CE the factions were private enterprises, run by those of equestrian status. In Rome their stables were located in the Campus Martius. Remarkably, the same four factions were found across the empire: Constantinople had the Blues and Greens, who nearly managed to bring down an emperor; this page gives an account of their internecine struggles. In Egypt, too, the Blue and Green factions rioted:
Chronicle of John Bishop of Nikiu, trans. R. H. Charles (London, 1916), p. 175.