Gandalf is white after returning from his battle with the Balrog in The Lord of the Rings. He first appears in the story as Gandalf the Grey, but after his death and return he becomes Gandalf the White.
Gandalf and the Balrog fall from the Bridge of Khazad-dûm into the depths beneath Moria. Gandalf defeats the Balrog on the peak of Zirakzigil, but he also dies from the struggle. His spirit is sent back to Middle-earth because his task is not complete.
The change is more than a costume change. Gandalf the White replaces Saruman as the chief opposing power to Sauron among the Istari, the order of supernatural emissaries sent to Middle-earth. He gains greater authority and rejects the limitations associated with Saruman’s betrayal.
The color names can be confusing because Tolkien also refers to other wizards, including the Blue Wizards and Radagast the Brown. Nevertheless, Gandalf’s defining transformation is from Grey to White, a symbolic and narrative sign of renewal and increased authority.