Overall, there are two major models of open access publishing: gold and green.

https://researchguides.library.vanderbilt.edu/c.php?g=144567&p=946137

https://www.openaccess.nl/en/what-is-open-access

There are two main models, or ‘routes’, to Open Access: the Gold route and the Green route.

Gold Open Access

In the Gold route the author, institution, or research funder pays an Open Access Fee (sometimes referred to as an Article Processing Charge or Book Processing Charge) and the publisher makes the published version free to read.

Funders have typically regarded Gold as the most desirable model of Open Access and, as such, when this blog is discussing Open Access it is primarily referring to the Gold route to Open Access.

Green Open Access

In the Green route the reader pays to read the publisher’s ‘version of record’, but there is also a manuscript (where the text is very similar to the published version but has not been typeset by the publishers) that is free to read and is available from a platform like an institutional repository.

RADAR is the institutional repository for Oxford Brookes from which the Green manuscripts of publications written by Oxford Brookes researchers can be freely downloaded: visit the Staff Publications section of RADAR. You will also find the versions of record published as Gold Open Access on RADAR.

Green Open Access is certainly a good model during a transition to Gold Open Access, but in the long run the integrity of the academic record is unlikely to be aided by having multiple versions of manuscripts available and, on a practical level, it can make the finding of an accessible version of a text even more complex than it already is through the ‘reader pays’ subscription model.

Other models of Open Access

Bronze Open Access

In the Bronze model no Open Access Fee is paid but the publisher chooses to make a publication freely available to read. Many Open Access advocates and research funders would not regard Bronze as truly Open Access because the publisher can stop the publications being freely available at any time (i.e. it relies upon the good will of the publisher), whereas genuinely Open Access publications have a specific licence (often a Creative Commons licence) that means the publication is irrevocably Open Access and the terms of use and reuse are clearly stated.

Platinum/Diamond Open Access

In the Platinum or Diamond routes the authors, institutions, or funders do not pay an Open Access Fee and the reader does not pay to read. In effect, this is a ‘publisher-pays’ model and is usually offered by university presses where the costs of publication are subsumed within existing budgets and regarded as part of the mission of a university. Platinum/Diamond is an excellent model for Open Access, but it is highly unlikely that it could replace the scale of commercial scholarly publishers in a reasonable timeframe.

Open Access publishing models

1. Gold Open Access

In this publishing model, the author must pay the publisher an Article Processing Charge (APC) while submitting the manuscript. Sometimes the author's affiliated institution or a funding body may pay this charge on their behalf. But unlike traditional journals, the reader won't be levied any sum to access or download the publication.

2. Green Open Access

This publishing model allows authors the freedom to submit and archive a version of their manuscript in a repository. The repository could be a discipline-specific one such as arXiv or the institution's repository that the author is affiliated to. Readers will access and download the entire content without paying any charges. If the manuscript has been published in a journal, the author will have to wait till the embargo period is over to archive it in the repository.

3. Diamond Open Access

This model involves the publishing of articles in journals that are entirely free. Neither the authors nor the readers have to pay any charges. Typically, these journals are community or institution-driven initiatives where they take the help of volunteers to process and put together the journal.

4. Delayed Open Access

It involves the publisher making the manuscript freely accessible to everyone on their website after the initial embargo period. Readers will be able to download the article without paying charges.

5. Hybrid Open Access

This publishing model is prevalent with traditional publishers where the authors have the option to make their individual articles (not the entire journal) immediately Open Access by paying an Article Publication Charge. The critical thing to note here is that once you pay the charge, you will get the article's copyright. The processing charge tends to be higher than that of Gold Open Access.

6. Bronze Open Access

This publishing model primarily refers to the practice of making articles widely available without having any open license attached to them.

7. Gratis Open Access

Here, the publisher has the option to make papers free to read at no charge to the author — usually for marketing and promotional activities. The Gratis Open Access may not be permanent. This is not considered as 'true' Open Access.

8. Libre Open Access

This is a blanket term for 'true' Open Access. The paper is made available under an open license, allowing it to be shared and reused, depending on which license is used.

source https://typeset.io/resources/open-access-publishing-ebook/#1-gold-open-access