Star System This wiki page here, details that purple indicates that the Crossbreed Campaign for Harlequin Warmbloods, has finally been completed and fully funded and in short, released into the game, much like how Elvish Warhorses are, National Show Horses, Appalachian Mountain Horses etc etc.

So, it's like the regular golden or yellow stars of purebreeds, but purple means it's been fully accepted into the game to breed and own, without the need for completing the campaign, but you still have to register future Harlequins to the club though, you just do not need to work towards a campaign for them any longer.