Probably purple. Many readers believe that Chapter 1128's color spread, which featured a purple Western dragon, foreshadowed Loki's fruit. Plus, base Loki also has a purplish color scheme.
Purple Dragon Loki leading Dorry the Blue Ogre and Brogy the Red Ogre works well, since mixing blue and red results in purple.
- Monsters' Dragon: Black
- PH's Dragon 13: Red
- PH's Dragon 21: Green
- Kaido's Dragon Zoan: Azure (Blue)
- Momo's Dragon Zoan: Pink
However, if the Hellfire Serpent mentioned in Harley and depicted in the mural as Loki's endgame opponent turns out to be the Red Line itself, then maybe green works. In color theory, the opposite of red is green. There could be two green dragons liberating the world: Loki and Monkey D. Dragon.