WARNING: The following contains spoilers for Legion of Super-Heroes #10 by Brian Michael Bendis, Ryan Sook, Wade Von Grawbadger, Jordie Bellaire, and Dave Sharpe on sale now
The Guardians of Oa are typically the ancient leaders of the Green Lantern Corps, giving commands to that force and helping spread justice across the galaxy. The Green Lantern Corps has come up against massive battles before, even against other Lantern Corps.
It appears that the Guardians of Oa eventually evolve into the golden energy-beings known as the Elders of Oa by the time of Legion of Super-Heroes, transforming in appearance and seemingly embracing the entire Emotional Spectrum as part of their mission.
The Guardians of Oa are among the oldest figures in the DC Universe. Dedicated to the protection of life across the galaxy, the Guardians are the ones who harnassed part of the power of the Emotional Spectrum and created the Green Lantern Rings. Since then, they have (for the most part) served as the leaders of the Corps, and served as its governing body. But it seems that their current form is not actually their final, as the Guardians will drastically evolve by the events of the 31st century.
After defeating Rimbor's Crav the General Nah during his outburst in front of the United Planets. the Legion of Super-Heroes agreed to take the former planetary leader to stand trial before the Guardians of Oa. Using his immense power, Gold Lantern is easily able to transport the General (alongside his teammates Brainiac-5 and Blok). But the Guardians are not much like the ones who commanded the Green Lantern Corps in the current era of superheroes. Now known as the Elders of Oa, the Guardians have evolved beyond their physical forms.
Instead, they appear to be conscious of golden energy, dancing like flames across their golden world. When someone is brought before them, they shine the "Light of Eternal Justice" onto the guilty -- seemingly locking them away in a strange and powerful container not dissimilar to the Phantom Zone Panes that various characters have been trapped within in the past. It's revealed that the Elders of Oa long ago realized the "greater purpose" of the Emotional Spectrum, and sought to unite it all behind the cause of bringing the galaxy a true sense of balance.
This would explain the Gold color scheme, as it could be seen as the pinnacle of the Green Lantern ideals, having come to take on the other elements of the Spectrum, all towards the collective growth of life. It's a fitting final fate for the Lantern Corps and makes the typical loss of the Green Lantern Corps within the time-frame of the Legion of Super-Heroes far less tragic. Instead, now the Green Lanterns don't exist because that organization -- much like the Guardians who command them -- have evolved into a new form instead.