In one hand, you have a white brass chip with a white middle, in the other hand a black one. Both hands are closed, and when opened, the white chip is now black, the black one now white. It becomes obvious to your spectator that you are only turning the chips over, that they are each black on one side and white on the other.
And that's when the magic happens. Put the chips in your hand, and remove the first one. It's not black or white- it's RED on one side and BLUE on the other! And when you reveal the other chip, it's GREEN on one side and YELLOW on the other! Startling!
Best of all, your hands are left COMPLETELY EMPTY- there is nothing left to hide!
By the way, this trick is also known as Black and White Surprise, or Chameleon Coins.