Wear Yankees gear or you'll get your ass kicked/mugged.
Throw another vote on the "This is stupid and you've probably never been to Yankee Stadium" pile. At one of the games I went to against the Tigers in 2007, a guy about 10 rows ahead of us had a custom Tigers jersey on, with what I can only assume was his last name on it (can't remember the name...something like Lennel). We were seated down the 3rd base line (where alcohol is allowed), and a guy a few rows behind us had a few too many, and just decided to pick on this guy.
EVERY time the Yankees would do something not great he'd start yelling "Lennel, this is all your fault!" or "Lennel, why are you doing this to us???" Sometimes, he'd even just yell out "LENNEL!!!" for no apparent reason. It was freaking hilarious. Wearing the opposing team's gear will without question draw attention to yourself. But it's no more of a "guarantee" of getting your ass kicked than walking down the street is a "guarantee" that you'll get run down by a taxi. Most people are there to have a good time.