Add Bootstrap class btn-danger for those 2 register buttons via jQuery.

Place this code in some JavaScript file that is loaded on site, or create custom one and load it:

jQuery('a[href*="individual-registration"], [href*="group-registration"]').addClass('btn-danger');

Dowload and install Add Custom Javascript from

Create the file above in a text file using notepad, name it as you wish and save with the extension .js

Upload the file to the Template js directory e.g. templates/protostar/js/registerbuttoncolour.js

Make sure the Javascript file path is added to where you have placed the javascript file e.g. templates/name of template/js/registerbuttoncolour.js

Publish the plugin

The Register buttons should now appear in red.