First check if your UI addon does this.
There are several font objects used in the UI you would have to find to change but the most common ones are GameFontNormal (GameFontNormalSmall, GameFontNormalLarge etc.)
Set GameFontNormal to green:
Code:
/run GameFontNormal:SetTextColor(0, 1, 0)
Other texts (especially in 3rd party addons) may use set fonts/size settings so you won't be able to change these "en masse"