You are allowed to use the official logo to present, test, review or promote Contao as a product. However, you are not allowed to use it for your own Contao products and services, because it would give the false impression that they were part of the official Contao project.
Own websites on the subject of Contao, which are not part of the official project, can use the Contao fanpage logo. This especially includes unofficial forums, community portals or international project websites. Contao partners are in addition entitled to use the partner logo, which is available in the partner forum or via e-mail.
You are never allowed to modify the logo in terms of color or graphics. There are four different versions: the official logo, the partner logo, the community logo and the fanpage logo, each in orange and in grey. You must not create further variations or modify an existing version in terms of color or graphics. Of course, resizing the logo is permitted.
Powered by Contao
If you have created a website with Contao and want to point out which CMS you are using, you are allowed to apply the logo to your site. You are presenting Contao as a product in this case and by adding the "powered by Contao" link you are also supporting the project.
Contao is being reviewed in a blog
In this case you are allowed to use the logo, because you are reviewing Contao as a product. It does not matter whether the article is published in a personal blog or a journal or whether it is positive or negative, as long as it includes a link or reference to the project.
Agencies offer their clients Contao
Of course you are allowed to use the official logo in this case, because Contao is presented as a product. A lot of agencies promote the CMS and eCommerce applications they are using to built websites for their clients and include their logos for this purpose.
A custom Contao extension is released
In this case you have to distinguish between the presentation of Contao itself and the custom extension. You are allowed to use to official logo to present Contao as a product, meaning the platform your custom extension is built upon. However, you must never use the logo for your own extensions, because it would give the false impression that they were part of the official Contao project.
A Contao related web service is established
Also in this case you are only allowed to use the official logo to present Contao as the product the service relates to. You must not use the logo for the web service itself, because it would give the false impression that it was an official service of the Contao project. Depending on the type of service, you can use the Contao fanpage logo though.
The Contao logo has been designed by Clemens Hackl.