Green in the Emperor's Eye

From publisher blurb: Thanks to heroic action in the arena, one of the Blues has been singled out for praise by the Emperor. Indeed, so pleased is the Emperor with this racer that an offer is made to let this Blue ride for the Greens for a race, as the Greens are currently in need of an extra pair of hands due to an injury. It is never said, but it is implied, that should the Blue run this race well, the offer may become permanent. Which would mean a spot on the team that has the favor of the Emperor. And that is a path to freedom from the arena. ...Only, racing for the Greens would mean betraying the Blues.