Green OA publishing refers to the self-archiving of published or pre-publication works for free public use. Authors provide access to preprints or post-prints (with publisher permission) in an institutional or disciplinary archive such as eCommons@Cornell and arXiv.org.
Hybrid OA offer authors the option of making their articles open access, for a fee. Journals that offer hybrid OA are still fundamentally subscription journals with an open access option for individual articles. They are not true open access journals, despite publishers' use of the term "gold open access" to describe this arrangement, and the Cornell Open-Access Publication Fund does not support open access fees to hybrid journals.
Diamond OA publishing describes journals that are completely free to publish and to read. The cost of maintaining and publishing the journal is usually borne by the organization that sponsors the journal. Diamond OA status has no impact on the journal's peer review process. By making articles completely free to both publish and to read, Diamond OA best approaches the goals of democratizing and widely distributing academic scholarship.
Bronze OA publishing describes articles that are free to read on a publisher's homepage, but without clarity on the specific licenses covering an article. Bronze OA articles may be free to read due to a temporary publisher marketing campaign, for example.