Green Open Access is self-archiving of the author's manuscript on a website or in a repository. The majority of publishers allow this, with some conditions.
Gold Open Access is posting the published article to a publisher website, with reposting allowed. This usually requires the author to pay an article processing charge, or APC.
Hybrid Open Access is a policy at many journals that allows authors the option of making their articles open access, if they pay an APC (Gold OA). If they do not pay the fee, the author manuscript may still be posted under the journal's "green open access" or self-archiving policy (Green OA).
Platinum Open Access is a practice by some publications that allows open access to its content without charging author fees. The publication may be sustained by donations or grants, or as an operational expense of the parent institution.
To learn about a journal or publisher's policies for self-archiving or open access, search the SHERPA/RoMEO database.