The cards show pictures with two objects that are not shown in the right colour, for example a green ghost and a red mouse: Here you have four details combined in a wrong way, green, red, ghost and mouse. The only object or colour not shown is the blue book. So the first to grab the blue book gets the card as a reward.
But be careful! Here is the twist: sometimes one of the items on a card shows a right combination of colour and object, i.e. a green bottle and a red mouse. The green bottle is a true object. So the first who grabs this item gets the card.
Everybody else who cached a wrong object loses one of his collected cards.
The game ends when the card pile has been used up. Whoever possesses the most cards wins.