Green with Envy

From publisher blurb:

The Emperor of Rome has long insisted that his favored team – the Greens – wins every race in every coliseum all across the Empire. However, political troubles have turned the Emperor's eye away from the more distant coliseums, and a few honest races have since been run in distant parts of the Empire. And with honest races, it happened: for the first time in many years at the coliseum of Alexandria, the Greens lost a race. Of course, the Greens did not take the sudden change in their fortunes well, and they have sent to Rome for one of their most famous racers, bringing him in to put these Blues back down in their place.