Green OA (or self-archiving) is the process of holding a version of a research output in a repository (like the UC Research Repository). Often publishers allow a version of the article to be held in a repository.
Gold OA applies when traditional journals publish articles that are free to read with no subscription barrier. Some journals are entirely OA, and a minority charge a fee (an article processing charge or APC) to cover running costs.
Hybrid journals charge subscriptions and charge an APC to make some articles OA.
