I have done some further investigation.
The last time I checked WP-SimpleViewer and jQuery Colorbox was a while ago so I have now checked the current versions of both (v2.2.0 and v4.2 respectively).
With previous versions of both plugins being activated together, the jQuery Colorbox plugin did not work at all and WP-SimpleViewer did not work when both plugins were used on the same page (though in different posts).
With the current versions of both plugins being activated together, things are a little different and look promising at first glance as both plugins function, even when both plugins are used on the same page.
However, in Firefox, the jQuery Colorbox's frame is hidden behind the WP-SimpleViewer gallery and in IE9, the entire jQuery Colorbox image itself is hidden behind the WP-SimpleViewer gallery.
Therefore, jQuery Colorbox is still incompatible with WP-SimpleViewer.
SimpleViewer Support Team.