A theory I have seen going around is that the order the orbs begin to glow and move around in the order the Elder Dragons awoke and began activity.
1. Orange, Primoridus, Elder Dragon of fire
2. Blue, the Deep Sea Dragon, Elder Dragon of water
3. Pale blue, Jormag, Elder Dragon of Ice
4. Poison green, Zhaitan, Elder Dragon of decay and undead
5. Purple, Kralkatorrik, Elder Dragon of crystal
6. Black (green glow), Mordremoth, Elder Dragon of the jungle and wood
The poison green orb slamming into the center orb is possibly after Zhaitan dies his magic and the magic he consumed being distributed back into the “roots” of Tyria or possibly consumed by the five remaining Elder Dragons.
(This is NOT my theory, but a collection of ideas I have seen on a video by Wooden Potatoes in both the comments section and some of it from the video itself.)