What is the difference between "Green" and "Gold" Open Access? What is the difference between Green Open Access and Gold Open Access?  Are there other "flavors" of Open Access?
The term "Open Access" has been used in numerous ways (see How Open Is It?), but in this context, Gold stands for publications available directly from the publisher, where as Green means that a version is available somewhere else (e.g. on an author's institutional website, or in a repository), a practice commonly referred to as "self-archiving". Note that neither of these two terms imply anything about reuse rights or about who shoulders the costs of publishing.

That colour spectrum has been expanded a bit to include Platinum, Titanium, White and Copper as well as Diamond, but these terms are used neither widely nor consistently. For instance, both platinum and diamond have been used to describe models where OA is provided by the journal and neither readers nor authors have to pay.