Yassine Gargouri, Vincent Lariviere, Yves Gingras, Les Carr, and Stevan Harnad have self-archived "Green and Gold Open Access Percentages and Growth, by Discipline" in ePrints Soton.
Here's an excerpt from:
Most refereed journal articles today are published in subscription journals, accessible only to subscribing institutions, hence losing considerable research impact. Making articles freely accessible online ("Open Access," OA) maximizes their impact. Articles can be made OA in two ways: by self-archiving them on the web ("Green OA") or by publishing them in OA journals ("Gold OA"). We compared the percent and growth rate of Green and Gold OA for 14 disciplines in two random samples of 1300 articles per discipline out of the 12,500 journals indexed by Thomson-Reuters-ISI using a robot that trawled the web for OA full-texts. We sampled in 2009 and 2011 for publication year ranges 1998-2006 and 2005-2010, respectively. Green OA (21.4%) exceeds Gold OA (2.4%) in proportion and growth rate in all but the biomedical disciplines, probably because it can be provided for all journals articles and does not require paying extra Gold OA publication fees. The spontaneous overall OA growth rate is still very slow (about 1% per year). If institutions make Green OA self-archiving mandatory, however, it triples percent Green OA as well as accelerating its growth rate.