Admin bar default color scheme for nonregistered/nonlogged users

Follow these steps:

1. Use the following code in your functions.php or your site specific
   plugin:

   function set_default_admin_color($user_id) {
       $args = array(
           'ID' => $user_id,
           'admin_color' => 'coffee'
       );
       wp_update_user( $args );
   }
   add_action('user_register', 'set_default_admin_color');
   

2. Now, add these lines to force “Coffee” color scheme to be displayed
   in the admin bar:

   if ( !current_user_can('manage_options') )
   remove_action( 'admin_color_scheme_picker', 'admin_color_scheme_picker' );