Colored Orbs

The Blue Orb, Red Orb, and Yellow Orb are items gathered by Matthew's party near the end of Golden Sun: Dark Dawn. These items are necessary to complete the game, although their true purpose isn't immediately obvious.

Acquiring the Orbs
Shortly after the Grave Eclipse begins, Volechek realizes that he hasn't restored an ancient weapon but rather unleashed a plague. To try and make amends for his mistake, he steals the Blue Orb from Blados and tosses it to his younger sister, Sveta, while she escapes Belinsk with Matthew. Quite some time later, Arcanus travels to Tonfon and gives the Red Orb to Emperor Unan, who then gives the orb to Matthew after he saves the emperor's nephew and neice. Matthew and his companions then return to Yamata and use the Red and Blue Orbs to explore the nearby ruins, where the Yellow Orb is stored.

Purpose
In addition to exploring the Yamata Ruins, the colored orbs act as the keys to Apollo Sanctum and the Apollo Lens stored within, which is the only thing able to stop the Grave Eclipse prematurely. The three orbs are used to open a gate atop the entrance of the Endless Wall, granting Matthew's party access to Apollo Sanctum. The orbs are later used to start up the Apollo Lens: The Red Orb brings out the weapon itself while the Blue and Yellow Orbs reveal the satellite dishes that collect the light used to power the lens.