How to display legal link on registration page in colorbox node?
You will have to add the class colorbox to the Terms and Conditions link generated by the Legal module.

This is easily setup through a theme_ function override.
Use the following code in your theme's template.php file:

    function YOUR_THEME_legal_accept_label($variables) {
      if ($variables['link']) {
        return t('&lt;strong&gt;Accept&lt;/strong&gt; &lt;a class="colorbox" href="@terms"&gt;Terms &amp; Conditions&lt;/a&gt; of Use', array('@terms' =&gt; url('legal')));
      }
      else {
        return t('&lt;strong&gt;Accept&lt;/strong&gt; Terms &amp; Conditions of Use');
      }
    }


Note: you might have to tweak the output in the popup, but that's outside of the scope I guess.