It’s a good time to be a Borderlands fan right now. The first installment of the series has officially made its way to the current generation of hardware, all previous titles received some enhanced visual updates, and Borderlands 3 has been announced and even has a release date. However, it appears that not everything Gearbox has planned for Borderlands 2 has made it into the game yet.
Twitter user "CaptainKody_," and the creator of the Reborn mod pack for Borderlands 2, decided to dig through the game's code after the recent updates. He discovered a new rarity for items which was added nearly seven years after the title’s initial release. As of right now, the Rainbow rarity is only within the game's files and not in the actual game. Being a modder and all, Kody decided to replicate the rarity on the weapons in the Reborn mod pack, and you can see the results in a Twitter post he made.
It's still unknown when Gearbox plans to add the Rainbow rarity to Borderlands 2, or if they will add at all. As Kody pointed out, there's a chance that Gearbox accidentally added it and they will be removing it in a future patch. However, it’s worth mentioning that the Rainbow rarity was not found in the files for Borderlands: The Pre-Sequel, so more Borderlands 2 content could potentially be on its way.