These cards originate from the continent of Fódlan, from three of the major nations in Fire Emblem: Three Houses. Not much is known yet as to what special skill or mechanic will be introduced to these new color until the game releases (more info below). As of Series 17, the leaders of each of the three main nations, Edelgard, Dimitri, and Claude, as well as their teacher, Byleth (Male), have been released.
It has been revealed for Series 18 that Byelth (Female) will be the featured in the new Starter Deck releasing at the same time. Series 18 (September 2019) and Series 19 (December 2019) will give a proper introduction of characters for the game.
At the time of writing this, the game will release on 26th July, 2019. More information will be added then.