Rainbow Road is the final chapter of Angry Birds Color Adventure . This is only in the HD Mode(Purchasable).
In the final chapter, they come to the Kingdom Of Colors, that dominated by pigs and Grey Enemies, except the Road From Rainbow, they have to follow to reach it.
They leave the Rainbow Road and should enter the castle, which is in ruins, to save the eggs.
At the end of the game, they rescue the eggs and finally defeat the Grey Enemies. As they prepare to take a beating in piggies, the black hole and sucks the boss right back to Piggie Island.