I would like to add the possibility to change color in CKEditor, mainly for Webforms. The instructions at https://www.drupal.org/project/colorbutton suggest to download manually the CKEditor colorbutton plugin and then add it to /libraries: would it instead be possibile to modify composer.libraries.json and install the plugin via Composer ? Thanks a lot
Yes. As Clive says in his comment above, you can certainly use Composer to manage other code besides Drupal core and contrib modules.
If you are keeping your code in version control such as
git, this also has the advantage of keeping third-party code out of your project's version control repository. Just commit your
composer.lock and you can rebuild all your project dependencies when you deploy.