I know what you have got by Dan Alex eBook
The mentalist takes out from his pocket a bag and says that inside there are three crystals. He explains to the spectator that he can mix them and pick one at random without looking, and put the remaining two in the bag, or choose of his own volition and then put the remaining two in the bag. This is all done with the mentalist looking away. Once the spectator has made his choice the mentalist will then tell the exact color of the crystal that the spectator has in his hand!