Reusing Piwik in any way is allowed as long as you respect the GPL license. If your product, software, or website uses Piwik 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 on your copy of Piwik.
If you distribute a modified version of Piwik, or software that includes Piwik as a module, or develop a website for a customer/client that integrates closely with Piwik, you must give access to the code source and publish all the code under the GPL.
Note that if you use third party plugins from the Marketplace, they may be released under a different license. Individual components, libraries or plugins within Piwik may also be available for reuse under a different license. Please refer to the LEGALNOTICE file included in the distribution.
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 Piwik with care. Please see the Piwik Trademark page for more details about how to reuse the name Piwik in your modified version of Piwik. Finally, if you modify Piwik and add new features or fix existing bugs, please submit your improvements back to the Piwik team: together we will build the best Free/Libre open analytics platform!