Gold Open Access
A direct publication as Open Access is called Gold. The author has to pay the publication fees as so-called Article Processing Charges (APCs). The publication is commonly published using a Creative Commons license.
Green Open Access
The secondary publication of originally not freely available titles is called Green Open Access. Usually, those titles will be published in a suitable repository.
Diamon/Platin Open Access
Directly publishing as Open Access without requiring authors to pay APCs is called Diamond or Platin Open Access. These publications are usually made through public institutions, such as universities, academies or expert associations.
Hybrid Journals do not normally publish Open Access article, instead they use regular subscription-based models in which the authors get an option of “buying” into Open Access by paying the APCs for their article to be published under a free license. The hybrid publication model brings the risk of “double dipping” costs where institutions end up paying for both the subscription fee as well as the APC for single articles.
Bronze Open Access
Bronze indicates publications on websites or repositories, which are freely accessible, but either lack any license information or use a non-free license instead of Creative Commons. The risk of using such publications is quite high, so only reading such publications should be considered free.