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(document).ready( function(){
jQuery('a[href*="individual- registration"], [href*="group-registration"]') .addClass('btn-danger');
});
Dowload and install Add Custom Javascript from http://joomla.impression-estudio.gr
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.