Why do I have to pay money to advance in the game as I do not have any purple diamonds left?

Yes, it is a free game. Players can earn diamonds without spending real money by exchanging regular bushes with other players. You can earn up to 98 diamonds per week if you exchange and receive 1 red, 2 blue, and 3 green bushes every day. Diamonds are the most valuable commodity and should be spent wisely. The game is playable without spending money, but it takes a little strategy and lots of patience.