Briefly the magus has a packet of four cards (nobody knows the color) and another one with four RED cards on the table.
He exchanges one card of his packet for one of the one on the table then shows that his packet has four RED cards
Then he exchanges two of his packet for two of the one on the table and then shows again four RED in his packet.
Now he exchanges three cards of his packet for three of the one on the table and he always have four RED cards.
Everybody think that all the cards are red but when he shows the packet on the table, the four cards are black.
If you play correctly your part, it's should be very funny.