At some point before the Second Great War, most of the guardians on Zhakul became corrupted by the hybrid Maar. Under Maar's control, they had a universal black and green color scheme. When Zeratul arrived on Zhakul to consult the preservers regarding the prophecy, he was forced to fight both Maar and the enslaved guardians.
Some of the protoss on Zhakul managed to escape Maar's control. During the battle, Zeratul rescued and recruited these survivors by restoring power to the relevant structures. Eventually, Zeratul freed the preservers and permanently destroyed Maar.