paths need to reference site or libraries path

For what it's worth, the addition of base_path() appears to fix the immediate problem I see:

public function getFile() {
    return base_path() . '/libraries/colorbutton/plugin.js';
  }

It doesn't appear that Drupal 8 supports a call to drupal_get_path('library', 'colorbutton') which would have been even better than assuming the library is at the root level. I libraries module provides an API to get the library path but that seems like overkill to add the libraries module for this.

That same approach needs to be applied to getButtons. For example, this is what I did:

  public function getButtons() {
    return [
      'TextColor' => array(
        'label' => t('Text Color'),
        'image' => base_path() . '/libraries/colorbutton/icons/textcolor.png',
      ),
      'BGColor' => array(
        'label' => t('Background Color'),
        'image' => base_path() . '/libraries/colorbutton/icons/bgcolor.png',
      ),
    ];
  }

For what it's worth, this same/similar change needs to be applied to pannelbutton as well.

I forgot to say, "Thank you!"

I think base_path(), but I'm not 100% certain. Doesn't DRUPAL_ROOT include the server director? For example, would DRUPAL_ROOT return "/var/www/drupal8"? If so, we don't want that because we're trying to create a URL and the URL would never reference "/var/www". We really just want the "/drupal8" such that, if using localhost without any subdomain, the URL would be http://localhost/drupal8/.... You'd never want a URL that was "http://localhost/var/www/drupal8"