What is 'Diamond' Open Access? I have heard of 'green' (self-archiving in a repository) and 'gold' open access (immediate open access in a journal), but what is 'diamond' open access? How does this differ from green or gold open access?
Diamond open access is like gold, in that the article is immediately open access in the journal, and nobody has to pay to read it.  However, in gold open access, the author (or their institution or funding agency) normally has to pay a publication fee to get the article published.  In diamond open access, they don't have to pay, so the process is completely free of charges to both authors and readers.

See http://www.jasonmkelly.com/2013/01/27/green-gold-and-diamond-a-short-primer-on-open-access/