A very clever trick based on an idea by Ken de Courcy. You show a bag containing four colored balls. You write a prediction and then let three spectators each get a ball from the bag (no force!). A ball remains. You get the last ball out of the bag, it is (eg) GREEN. The prediction correctly states that the last remaining ball will be GREEN!
You get everything complete with German instructions, bags and balls. Incidentally, the bag is not a traversing bag, but a bag that you can also have examined.