Reusing Matomo (Piwik) in any way is allowed as long as you respect the GPL license. If your product, software, or website uses Matomo but all the code (including your modifications) stays within your company or server, you have the freedom to keep your code and not be bound to the GPL License. For example, you can change the header and footer to reflect your brand, change the colors, or add new features and plugins on your own copy of Matomo.
If you distribute a modified version of Matomo, or a software that includes Matomo as a module, then you must give access to the code source and publish all the code under the GPL.
If you create and distribute a new custom plugin built for Matomo, as a Plugin developer it is allowed to release the plugin under a different license than GPL, as long as the plugin is distributed separately from the Matomo platform. For example some third party plugins from our open Marketplace may be released under a different license than GPL. Similarly individual components, libraries and plugins within a Matomo instance may be available for reuse under a different license. Please refer to the LEGALNOTICE file included in the distribution.
When you resell Matomo to customers, you may be interested in our White Label product which removes all the branded aspects of Matomo and lets you provide an immersive experience and your own custom brand. You may also be interested in reselling the Matomo Cloud
You may want to learn more about the GPL License and look at the GPL FAQ. As well as respecting the GPL license, you must also reuse the word Matomo with care. Please see the Matomo Trademark page for more details about how to reuse the name Matomo in your modified version of Matomo. Finally, if you modify Matomo and add new features or fix existing bugs, please submit your improvements back to the Matomo team: together we will build the best Free/Libre open analytics platform!