Open access (OA) literature is a method of sharing scholarship that is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes OA possible is the internet and the consent of the author or copyright-holder.”

--From A Very Brief Introduction to Open Access by Peter Suber

Types of Open Access

There are three main types of Open Access. 

• Green– alludes to self-archiving generally of the pre or post-print in repositories
• Gold– Open access peer-reviewed journals, are freely available on the internet. Some publishers charge article processing charges (APCs) or fees (APFs). See the Directory of Open Access Journals (DOAJ).
• Hybrid– sometimes called Paid Open Access, refers to subscription journals with open access to individual articles usually when a fee is paid to the publisher or journal by the author, the author's organization, or the research funder. Some of the fees are quite expensive, up to $5000. Some universities or libraries have a pool of funding available for hybrid journal publications or sometimes funding is written into grant applications for open access in hybrid journals, though these are not common instances. Some examples of hybrid open access are: iOpenAccess by Taylor Francis, Online Open by Wiley, or Sage Open by Sage. For a full list visit Publishers with Paid Options for Open Access from SHERPA/RoMEO.
• Diamond- Open access scholarly publishing free of fees and access charges, for example, Discrete Analysis, an online peer-reviewed journal where articles are links to arXiv

Though green open access generally refers to the post-print of an article, there are three basic version types that can be self archived in repositories:

• Pre-Prints – The author's copy of article before it’s been reviewed by the publisher, or pre-reviewed
• Post-Prints – The author's copy of article after it’s been reviewed and corrected, but before the publisher has formatted it for publication, or post-reviewed.
• Publisher’s Version – The version that is formatted and appears in print or online.

If authors have signed a Copyright Transfer Agreement (CTA), publisher's policy will determine which version of an article can be archived in a repository. Most publishers allow some sort of green open access. Authors can check their CTA for this information. SHERPA/RoMEO is a database of publisher copyright policies and self archiving information that authors can use to check which version they may be allowed to archive. Not all journals are in SHERPA/RoMEO and it isn't always current, so authors may also want to check the publisher's website as well.

SHERPA/RoMEO classifies publishers into colors for easy identification:

• Green- refers to publishers whose policies allow archiving of pre-print and post-print or publisher's version/PDF
• Blue- refers to publishers whose policies allow archiving of post-print or publisher's version/PDF
• Yellow- refers to publishers whose policies allow archiving of pre-print
• White- refers to publishers whose policies do not formally support archiving any version