Since there wouldn't be any upgrade to the weapon, it could be as simple as the weapon moogle saying "Oh, we've been researching weapon glow effects. We have found a way to make ergon weapons glow" <moogle magic animation/cs> "Speak with Oboro, something something - kupo"
Trade your weapon to Oboro, he says give me 1000 beits (arbitrary number), wait a day, green glow. Green is a component of the RGB - Adoulin storyline is all about nature - so that seems kinda appropriate.
There is already a green glow existing in the environment, so all they would need to do is add some dialog and a quest. It all comes down to how important it is, and it isn't really a functional asset so much as it is
neat thing.