What the colours actually mean
Open access is not a single thing. It's an outcome — research anyone can read, free at the point of use — that can be reached by several different roads. The publishing world has settled on a colour-coded shorthand to describe them. Green, gold and diamond are the three you'll encounter most often, and understanding the difference matters whether you're a researcher deciding where to publish, a student hunting for free papers, or a funder trying to write sensible policy.
Green open access means self-archiving: a researcher deposits a version of their work in a repository — an institutional archive, a subject-specific one like arXiv or PubMed Central, or a general-purpose platform like Zenodo — separate from wherever the journal publishes the final article. The paper in the repository is typically a preprint (submitted but not yet peer-reviewed) or an accepted manuscript (peer-reviewed but not yet typeset by the publisher). The polished version of record usually stays behind a paywall, at least for a while. Most journal publishers now permit some form of green deposit, though embargo periods — a waiting time before the repository copy can go public — can stretch from six months to several years depending on the publisher and the discipline.
Green is the most widely available route and costs nothing directly. Its catch is a version problem: the copy readers find in a repository may differ from what finally appeared in print, and navigating publisher policies about which version can be deposited, and when, can be genuinely confusing.
Gold open access means the final published article is immediately and freely available on the journal's own website, usually because someone has paid an article processing charge — an APC — to make it so. APCs vary wildly: a few hundred dollars at some titles, several thousand at flagship journals from major commercial publishers. For well-funded researchers at well-resourced institutions, APCs are often covered by grants or institutional agreements. For everyone else, they can be a real barrier — effectively moving the paywall from the reader to the author. Some publishers offer waivers for researchers in lower-income countries, but these are not always easy to claim or consistently applied.
Within gold, there's a further distinction worth knowing. A fully open-access journal makes every article freely available. A hybrid journal is a subscription journal that sells individual articles as open access for an extra fee — a model critics argue allows publishers to collect revenue twice, from both subscriptions and APCs.
Diamond: the third way
Diamond open access — sometimes called platinum — is gold without the APC. The journal publishes everything freely online and charges authors nothing. The costs of running the journal are covered instead by institutions, learned societies, libraries, or direct grants. The Directory of Open Access Journals (DOAJ) lists thousands of titles operating this way, a significant portion of which are diamond. Many are smaller, specialist publications run by academic communities rather than commercial publishers, and for those communities they work extremely well.
Diamond is in some ways the model that most closely fits the original vision of open access: no paywalls for readers, no fees for authors, scholarship circulating freely. Its limits are practical — funding a diamond journal requires sustained institutional will, and not every field has the infrastructure to support it.
Choosing a route
In practice, the route available to a researcher depends on a tangle of factors: what the publisher allows, what a funder mandates, what an institution can cover, and what the norms of a discipline are. Physicists and mathematicians have posted preprints to arXiv for decades; in some humanities fields, the practice is still unusual. Funders — public research councils, private foundations — increasingly mandate open access and specify which routes they'll accept.
Creative Commons licences are the legal machinery that makes open access usable, specifying what readers can do with what they find: cite only, adapt, remix, use commercially. A paper that's free to read but carries no clear licence sits in a murky middle ground.