1983 to 2009 New Era Hat Tagging Primer

[joelsabi]

The first team to switch to gray undersides were the Reds in the 1960s. The Giants were one of the next teams to switch, and then the White Sox and a few others. The last NL hold-out (I believe) was the Cubs, going gray in 1994. The Blue Jays, Twins, and Yankees held out until the 1995 season. The switch for all remaining teams with green to switch to gray in 1995 was insisted by New Era.

The Angels were the first team to change to black, around the late 90s. 2007 was when all teams changed to black.

underbills is another factor. To add to this, the Pirates were the first to use red in 2001.