Conditional Simple Colorbox is a WordPress plugin (download) that adds a Colorbox to footer of your site only when needed.
When you use Simple Colorbox, it loads Colorbox files on each page. This plugin changes that and loads Colorbox files in footer of page only when there are images or gallery in post content.
Conditional Simple Colorbox is a very lightweight, it has no settings, just activate it and it works immediately.
Conditional Simple Colorbox is released under the terms of GNU General Public License. It is completely free so if you find it useful you can give something back to author. Your help could be done by giving donation, sending pull request on GitHub, sending bugs and suggestions, promoting plugin in blogs, forums, social networks, and other websites.