New York
The company that makes the contacts, Sports Tints, claims that they boost performance by aiding a player's ability to focus on certain colors and objects.
The amber color, for example, supposedly helps
NBC in Miami did a last year report on a bunch of athletes in Florida who swear by the contacts. And in the past few months we've seen baseball players, football players, and even golfers wear the lenses in competition.
Bryce Harper looks to be one of the advocates:
Youtube Screen Grab
Brandon Inge of the Pittsburgh Pirates was seen wearing the yellow lenses:
Youtube Screen Grab
It's now starting to show up in the college ranks as well:
ESPN
Like other "alternative" performance aids, there is no definitive study that proves that these things work.
A study performed in 2007 in the of journal Optometry by 35 college and professional football players, even showed that the contacts are not clinically significant enough to improve athletic performance.
Despite this, it seems players still use them to find a new competitive edge against their opponents. Even if it's a placebo effect, it's worth it for these guys.