"Revert to Google's 2013-2015 Favicon" (https://greasyfork.org/en/scripts/12271-revert-to-google-s-2013-2015-favicon
) works simply, well, and accurately - it targets ONLY those Google sites that got the new favicon on Sept 1, 2015. Google has about 50 different sites, most of which don't use the "Google Search" favicon (did not get the new favicon). The "Revert to..." script uses fine-tuned "@include"
and "#exclude" URLs (with wildcards) in its metadata block to correctly target or exclude ALL Google sites, so as not to change "non-Search" (site-specific) Google favicons. (This comment was posted by the author of the mentioned script).