It's not entirely noticeable, but it's definitely pink.
The LA Times reports Williams' mitt is a tribute to his mother, whom he lost to cancer in 2001. He got the glove while pitching in Taiwan. Williams wore it in Triple-A, but then stashed it away when he moved up to the majors.
The Times reports Williams' cleared it with the umpires during a September 2011 game against the Mariners. He gave up only one hit over eight innings and has worn it ever since.