It would be totally against the laws of nature to wear a Red Sox jersey to Yankee stadium. Especially during a game. No danger of a beating I don't think but the ugly stares and the throngs of people cursing you and your ancestry would be a bad thing.

It's fine- if you want to bring along an exorcist and a bunch of spellcasting or scimitar wielding characters from a Dungeons & Dragons game...

I've been to a few games at Yankee Stadium. 

I'm a Red Sox fan and I  loathe the Yankees. 

I usually wear a neutral, non-baseball cap when there.  On one occasion, I arrived for a game on a brilliantly sunny day having forgotten a cap.  Unwilling to risk sunstroke, I went down to the concession stand, bought the cheapest Yankee hat I could find, and dropped it in a garbage can on my way out of the stadium after the game.

 Some principles just aren't worth taking that big a risk for.

Almost as dangerous as wearing a George Clooney t-shirt when you are hunting with Dick Cheney!

It is kind of bs that in the world of spectator sports we should be cautious about cheering for our team at another team's venue, but you have to see it for what it is.  Would wearing a Red Sox jersey there really cause you to catch a beating?  Odds are strongly against it, but personally I wouldn't do it, or loudly support my team in any harsh visitor environment.  Why?  When I was about 17 I was there cheering for the Sox and I got hit in the head with a water bottle, true story.  Just a water bottle but it made me think, some dick could throw something at you and if you ran up in Ron Artest fashion you would probably be in for a world of trouble.

I'm sure it's also against the law in New York to wear a Red Sox jersey...period.