While the Reds were the first team to wear the green on St. Patrick's Day in the 1970s, it was that 1990 Classic card with Mike Schmidt in the middle there that showed collectors the uniform possibilities of the 17th of March.
Over the years, it has become a tradition for teams to wear green today (except for the A's who almost always wear it no matter what city they are in). And hey, even I am wearing my green shamrock-clad David Wright shirtsy today, lest I chance getting pinched.