Bobbleheads, beanies and even Rubik’s cubes have become popular Major League Baseball collectibles in recent years. What’s another trend in MLB merchandising? It’s actually a color: green. Using St. Patrick’s Day as a kickoff, MLB franchises have launched promotional campaigns around a wide array of logoed green T-shirts, jerseys, caps and cell phone cases.

While green is in vogue today, its MLB marketing roots date way back to 1978. Dick Wagner, then the general manager of the Cincinnati Reds, surprised his team when he ordered custom emerald green uniforms, caps and catcher’s gear. No one had any idea what Wagner had done until game day, when green uniforms were hung on each of the player’s lockers. From head to toe, any part of the uniform that was traditionally red was green – for one March day. It was unorthodox, to say the least.

Fast forward more than 30 years and about a dozen franchises featured special green-colored merchandise this past season, according to an MLB spokesperson. Some teams kept it simple, with green caps or batting gloves. Others, like the New York Mets, wore green jerseys and used shamrock bases on the field. The Cincinnati Reds went as far as to serve green beer at a game.

The Boston Red Sox, whose Fenway Park features a leftfield wall known as the Green Monster, have been wearing themed hats since 1990 on St. Patrick’s Day. The Sox first wore green jerseys in 2004, when they hosted the Cleveland Indians at City of Palms Park – the club’s one-time spring training home – in Fort Myers, FL. Majestic Athletic designed the green jerseys, which have become a popular St. Patrick’s day tradition for fans of Boston, well known for its strong Irish heritage.

Besides selling themed jerseys and hats at select games, the Red Sox are one of several teams to offer green merchandise online, with a host of items geared toward women. This year, Boston will auction off some of its players’ alternate jerseys and donate the proceeds to the Red Sox Foundation.

Meanwhile, the Detroit Tigers, hailing from another city steeped in Irish tradition, once had its bat boy dress as a leprechaun at a game. And the Philadelphia Phillies’ team mascot, the iconic Philly Phanatic, has dressed up like a leprechaun and danced the Irish jig for fans in Clearwater, FL.

Over the years, these special uniforms and promotional pieces have brought in green – as in money – particularly through themed promotions. Although MLB and its franchises haven’t released a specific dollar figure, the expansion of green branded gear demonstrates its value to teams’ continued fan programs. The green trend has absolutely turned into an annual marketing and sales opportunity for clubs. “We will have headwear, T-shirts and the Reds’ St. Patrick’s Day authentic batting practice jerseys – just like what the players will be wearing on the field – available for sale,” says a club spokesperson. “The merchandise is festive.”

It seems Dick Wagner was onto something after all.