The game is played in teams. The teams are re-assigned randomly every game. This depends on the red 2s (= "Partner-cards"), because all players holding a partner-card at the start of the game form a team against all of the players who don't have a partner-card. Don't tell anyone that you have a partner-card - keep it secret. No one knows who their partner(s) are.
You become rich by collecting diamonds: The more the merrier. At the end of the game both teams count their diamonds. Depending on the amount of players, the required amount of diamonds to win varies. Check below under set-up.
Be rich. If not in real life, at least in a game.
... is played in rounds.
One round consists of every player placing one card, one of which will win the round. The winner then starts the next round.
The first card of each round is freely chosen and determines which cards the other players are allowed to play. You have to play a card from the same colour group as the first placed card every round. For example, if the first person plays a blue card, you can place any of your blue cards, ... (and so on).
If you happen to not have a card of the group in question, it is up to you to choose any other card from your hand.
Once every player has played one card each, the highest card will win the round. The player who won the round get's to keep the cards on their own discard pile.
The saying is: The lower the number, the higher the value of the card. For example, the number 1 is the 1st and best card of its group. The number 2 the 2nd best, ... and so on.
Red cards are always superior to all cards of other colours (If you have played other card games that have 'trumps', then this is the same idea - the red cards are the 'trumps').
If all the cards on the table are non-red, then the first placed card determines the winning colour for that round. Within that ‘winning colour’ the card with the lowest number wins.
For example, suppose that the first card that is placed is a green card. The other players must also place a green card if they have one. If they don’t have any, they can place a card from another group (such as red), but that card will not win the round. The winning card will be the lowest green card (since that is the group that started the round).
Win a trick with the thief and steal another player's diamond.
Win a trick with the merchant and swap a card from your pile and hand.
The second magician trumps the first.