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.
A diamond open access journal can be paid for through the Society's membership fees, which include access to the Journal. If journal access is free anyway, you may ask why would people pay membership fees? Because they believe in the mission of the Society, want to support the open access journal, get a discount on annual meetings, and generally support that society and journal.