Drupal Colorbox with Prezi iframe
You created your link by concatenating width=500&amp;amp;height=500&amp;amp;iframe=true with an ? instead with another &amp;amp;
So the working link would be:

&lt;a class="colorbox-load" href="http://prezi.com/embed/id-of-the-file/?bgcolor=ffffff&amp;amp;lock_to_path=1&amp;amp;autoplay=0&amp;amp;autohide_ctrls=0&amp;amp;features=undefined&amp;amp;disabled_features=undefined&amp;amp;html5=1&amp;amp;width=500&amp;amp;height=500&amp;amp;iframe=true"&gt;open in Colorbox (iframe)&lt;/a&gt;