Self-archiving or publishing in an institutional or subject repository, website, or scholarly collaboration network.
Publishing in Green OA is free of charge, but most publishers will impose an embargo period before allowing OA publishing of the article, or allow sharing the pre-print version of the article.
Bronze Open Access
Making the publication available on the publisher's website free of charge, but with no license information and for an undefined period of time.
The problem here is that the users do not know for how long the work will be available freely online, and sharing or reusing the work is restricted or not possible due to the missing of the license information.
Gold Open Access
Publishing in Open Access or hybrid journals, usually with copyright restrictions (CC license).
The author may pay an article processing fee, but there is no embargo period limitation, and applying a CC license enables the author to determine how others may reuse or share the work.
Black Open Access
Refers to illegal copies of the publication deposited in illegal repositories violating the publisher copyright agreement.