Black Shackle

All parts are examined: a black recording device with four perforations, a yellow and a blue disc with four perforations, and a black silk ribbon.

Now the two differently colored perforated discs are tied four times to the recording device by pulling the black tape through the holes.
> It is obviously impossible to free the discs without passing one end of the tape back through the holes!

You place this arrangement on the outstretched hand of a spectator, who may now name a color.

As soon as he has done this, pull up the tape and with it the recording device:
> And the chosen color disc lies freely in the spectator's hand!