Indeed surprisingly hard to find a first hand reference. But http://duckduckgo.com/?q=%22painted+blue%22+expansion returned link to wikipedia article, which, in turn, links to some meta-document about new C standard. There's also some post on Boost (library) mailing list: http://duckduckgo.com/?q=macro+%22painted+blue%22+boost -> http://lists.boost.org/Archives/boost/2006/01/99264.php

Perhaps I was wrong; perhaps the phrase was in K&R C? Can't find the book online @_@;;