Personal speculation with added "lore" Crag - red - rocky iron to crush enemies Solar - orange - in game as is Germ* - yellow - poisons foes who are biological and heavily armored. Draught - green - deals more damage to light weight enemies, can manipulate enemy positions on the field, knock back, push etc Arc - blue - as is. Aqua - indigo - higher damage to mechanical enemies. Void - violet - as is. Rime - white - can freeze, slow or stun targets. *Or a suitable name for poison So the traveller is a light bearing spheroid with seemingly magic powers, it's almost dead but can still give millions of guardians the strength to push back the darkness. How does it do this? Built-in solar batteries. Not solar panels, solar batteries, suns inside of batteries condensed to fit all 8 known sun types. Each battery has a different sun composed of gas infused plasma, over time the systems were damaged through war and traveling. Now the cores of each sun are decaying and that decay results in slightly weaker powers but the powers have actual traits that make them versatile. We would need a considerable amount of resources to repair the traveller and gain increased power. Helium, Nitrogen, Oxygen, Copper, etc... Maybe we never fix the traveller, maybe our powers fully decay into their respective "elements".

It seems to me that the game was build with that in mind

It would be cool for the warlock to summon arc energy and shoot any enemy close to each other while arking through the enemy like the Wunderwaffen DG2 in World at War. The titan could have a solar punch that he can throw and have area effect while the hunter could have a void heal where he can shoot other guardians and they either replenish health and any kills they get make light for the hunter or links the guardians together making them all create orbs of light for each kill they get.