| Gold OA | Journals do not charge subscription fees and authors pay the article processing charges (APC). |
| Green OA | Also known as self-archiving, authors deposit pre or post-prints to an OA digital repositories and pay no fee (with 6-24 months embargo). This can be an institutional or a disciplinary repository such as PubMed Central. |
| Hybrid OA |
A subscription journal in which some of the articles are open access. It is mixed revenue model of subscription charges and pay-to publish options. Authors pay a subscription-based journal a publication fee to make their individual article open access immediately upon its release. Hybrid OA remains significantly more expensive than full OA (~50% more per APC). |
| Bronze OA | Delayed OA. Free to read on the publisher’s website. The publisher controls copyrights. |
| Platinum/ Diamond OA |
Free to the authors and free to the readers. Usually sponsored and published by nonprofit societies and associations, e.g., the Beilstein-Institut and the Electrochemical Society. |
There are Two Tiers of Open Access Publishing, Gold and Green.
GOLD Open Access - Authors can submit their works to a Peer Reviewed Open Access Journal for publication. 70% percent of Open Access Journals require no-fee. Costs for publication can often be paid by sponsoring institutions, scholarship dollars and support from departments and schools.
Here is a listing of the Open Access Journals, as provided by the Directory of Open Access Journals (DOAJ)
GREEN Open Access - Authors make their work freely available in a Digital Repository without need for Peer Review. These repositories do not provide peer review, but can still store content that has been peer reviewed. Repositories are not limited to traditional scholarship and can hold video, data sets, institutional records.
Publishers can deny access to the majority of open-access articles at their discretion.
"Most open-access articles are not accompanied by a license, severely curtailing their use, a recent survey of 100,000 articles sampled from the CrossRef database has revealed. Without a license, articles are free to read, but can’t be redistributed or reused, for example, in presentations or course material."
"Without explicit permission, they also can't be mined by computer software. “As artificial intelligence and machine learning become increasingly important tools, we need our research literature open and available for computational approaches to synthesize it, summarize it, and discover new patterns.”
https://www.nature.com/nature-index/index.php/news-blog/bronze-open-access-supersedes-green-and-gold
The green/gold distinction is about venues (repositories and journals), while the gratis/libre distinction is about user rights or freedoms. Green OA can be gratis or libre, but is usually gratis. Gold OA can be gratis or libre, but is also usually gratis.
It’s easier for gold OA to be libre than for green OA to be libre. Both both can be libre. It follows that the campaign to go beyond gratis OA to libre OA is not just about journals (gold OA), even if it is primarily about journals.
https://sparcopen.org/our-work/gratis-and-libre-open-access/