What is the difference between Green Open Access and Gold Open Access? Are there other "flavors" of Open Access?
The term "Open Access" has been used in numerous ways (see How Open Is It?), but in this context, Gold stands for publications available directly from the publisher, where as Green means that a version is available somewhere else (e.g. on an author's institutional website, or in a repository), a practice commonly referred to as "self-archiving". Note that neither of these two terms imply anything about reuse rights or about who shoulders the costs of publishing.
That colour spectrum has been expanded a bit to include Platinum, Titanium, White and Copper as well as Diamond, but these terms are used neither widely nor consistently. For instance, both platinum and diamond have been used to describe models where OA is provided by the journal and neither readers nor authors have to pay.
Expanding a bit Daniel's answer.
Gold and Green are traditional terms used by the OA community, focused on the how open access is implemented, how the research is provided to the final users. It's a single dimension of OA, and these terms do not say anything about others dimensions. PLOS How open is it? leaflet does a great job in splitting the term "open" in several dimensions:
- reader rights (there is an embargo?)
- reuse rights (what is the license of the research? CC-BY, CC-BY-SA, or CC-BY-NC-ND?)
- copyrights (does the author retain copyright, or it's the publisher?)
- author posting rights (can the author archive the research in other repositories or websites?)
- machine readability (is it a PDF or a structure XML file with research data attached in proper formats?)
Moreover, you could add
- costs of publishing (who pays for publishing the article? the author or the institution?)
- peer review (is the article peer reviewed or not?)
The peer review factor has been probably the single biggest difference between Green and Gold OA, there are many exceptions.
Traditionally, if you uploaded a pre-print on an institutional or disciplinary archive, that was green OA; but if you wanted published in a proper journal (thus, with peer review), that was called gold OA.
But very often you are allowed to take the published article and upload that on an repository: that it's still green OA, but the article is peer reviewed.
So, in the end: open access is complex, because there are many dimensions in how research is written, published, payed, accessed. Gold and green are traditional terms that focus on peer review and where the research is published.
We have a section on the 'types' of open access in the Open Research Glossary. Daniel's answer is how I think most in the open access community (a much better term than 'activists') would describe green and gold OA. However, they have been largely 'hi-jacked' by publishers to mean different things. Gold OA is often equated with an 'author pays' model of access, whereby an article-processing charge (APC) is levied to conform to the definition provided by Daniel. Green OA, on the other hand, is often used to describe an item in a repository subject to an embargo period, enforced by the publisher (this can all be checked on a journal-by-journal basis using the Sherpa/Romeo tool).