The Two Towers by J.R.R Tolkien (Harper Collins 1991, 2007) pp. 644, 645
We can be sure that if the mysterious old man who climbed up the hill upon which Merry and Pippin first met Treebeard was indeed Saruman we would now be subjected to a very long speech. It would be a speech about his greatness, one intended to fill his hearers with awe, but all Gandalf says about himself and his transformation is to say:
“Yes, I am white now,” said Gandalf. “Indeed I am Saruman, one,might almost say, Saruman as he should have been. But come now, tell me of yourselves!”

When Gandalf was imprisoned by Saruman in Isengard he was subjected to such a speech. “We must have power,” Saruman said, “power to order things as we will, for that good that only the Wise can see.” Saruman was anxious, not only to subject Gandalf to his will but to convince him that he had the right to be the Lord of the Rings and thus Lord of Middle-earth.
From the beginning of the mission of the Istari, the wizards, to Middle-earth, Saruman was anxious that he should be its leader. And when with Gandalf, Galadriel and Elrond he formed the White Council, a council of the Wise to oppose Sauron, he insisted that he should be its leader even though Galadriel argued that the leader ought to be Gandalf.
Although Gandalf never sought power for himself Saruman was always jealous of him and looked for ways to undermine the one who he believed to be his rival. So he made fun of Gandalf’s affection for hobbits and the Shire while beginning to forge links between the Shire and Isengard; and he mocked Gandalf’s enjoyment of pipe-smoking and of pipeweed, while secretly learning the art himself and purchasing the best of Longbottom leaf from Lotho Sackville-Baggins who became his agent in the Shire.
But most importantly of all Saruman believed that Gandalf was his rival in seeking to find and to take possession of the Ring. Like Sauron he was convinced that if anyone of sufficient strength were to find the Ring they would claim it for themselves and use it to become the ruler of all. And he became convinced that Gandalf was trying to find the Ring just as he was so that he should become lord of all and that when he began to suspect that the Ring was hidden in the Shire that the same hobbits who he had despised were being used for some obscure purpose in Gandalf’s plot.
All Saruman’s suspicions were, in his mind, confirmed when he and Gandalf met once again in Isengard after the Battle of Helm’s Deep. Gandalf demands that Saruman surrender the Key of Orthanc to him and his staff as pledges of Saruman’s good conduct and to be returned later to him if he should once again merit them. Saruman responded to Gandalf’s demand with undisguised rage.
“Later!” he cried, and his voice rose to a scream. “Later! Yes, when you also have the Keys of Barad-dûr itself, I suppose; and the crowns of seven kings, and the rods of the Five Wizards.”
Saruman was utterly convinced that Gandalf desired what he himself did, that Gandalf was his rival and therefore his enemy. And perhaps he feared that he was his enemy’s inferior, that Gandalf possessed a power that he himself lacked, and that he needed to surround himself with a fortress, an army and all the trappings of power in order to be what Gandalf was, in himself, alone, vulnerable and homeless in the world. And so he became unsatisfied with his white robes and made a coat of many colours for himself. There is a sense in which he gave up his white robes quite voluntarily having become unsatisfied with what they represented, that is that he was an emissary of the Valar in Middle-earth. That these robes should be given to Gandalf, the very one that he feared and hated most, only confirmed what he always believed, that Gandalf desired to rule just as he did.

What he had forgotten, indeed despised, was that his power and status did not belong to him but had been given to him in order that he might be an emissary of the Valar in Middle-earth. His task was to do the bidding of his masters and so when he proved unfaithful in doing that task his masters stripped him of his robes and gave them to one who would do their bidding. Gandalf is now the White, Saruman as he should have been.