How to display an overlay image in modal window (Colorbox) upon hover
You would probably have to write some custom javascript / jquery in your theme's JS file and call the colorbox function on hover. For example:

$('.img-1').hover(function(){
  $('.img-2').colorbox();
});


That's only the starting point. Once it is working with just 2 images, the next step would be to dynamically pair all the images so the javascript knows which image to show on hover.