Like with ballpark seat colors, green is by far the color of choice in major league ballparks. More than two-thirds of the 30 major league ballparks have green walls.
Twenty One MLB parks have green walls: Angel Stadium, Fenway Park, Rangers Ballpark, Target Field, Comerica Park, Safeco Field, Tropicana Field, U.S. Cellular Field, Kauffman Stadium, Progressive Field, Oriole Park, Citizens Bank Park, Coors Field, Busch Stadium, AT&T Park, Great American Ballpark, Minute Maid Park, Chase Field, PNC Park and Nationals Park.
Blue walls are the second-most used in MLB. Unlike green though, there is actually variety of shades of blue in use. Yankee Stadium has uses a navy blue color. Turner Field, Rogers Centre, and Miller Park have more of a royal blue color. Dodger Stadium seems to have almost a powder blue color. The walls at Petco Park in San Diego are a very very dark blue. (***I'm not 100% sure of this, it is possible that they are black.)
Citi Field is the lone ballpark in the majors with black walls. While many Mets fans feel they should revert to the royal blue color that was used at Shea Stadium, I find the black walls to be striking and unique. Whenever you see the black walls in a highlight package, you instantly know what park it is. In a league where 90% of the ballparks use green or blue walls this exception is refreshing. While I think the Mets should wear their traditional blue caps more (especially with the road uniforms), I hope the black walls stay for a long time.
In my opinion one of the best traditions in sports is the brick walls covered in ivy at Wrigley Field. While they're somewhat unsafe for outfielders early in the year before the ivy grows in, its a unique feature of the second oldest park in the majors.
Finally, we have the teal walls of Sun Life Stadium. While the Marlins don't use much teal in their uniforms anymore, they have chosen to keep the teal walls throughout their stay at this multi-named multipurpose stadium. Looking at the first renderings of their new retractable-dome that will open in 2012, the Marlins seem to be ready to join the green wall majority in a few years.