We know that Saruman bore ill-will toward Gandalf for millennia before the events of The Lord of the Rings. It began with their arrival in Middle Earth when Cirdan gives Narya to Gandalf, or at least shortly thereafter when Saruman learns of this gift.
And the Grey Messenger took the Ring, and kept it ever secret; yet the White Messenger (who was skilled to uncover all secrets) after a time became aware of this gift, and begrudged it, and it was the beginning of the hidden ill-will that he bore to the Grey, which afterwards became manifest.
The manifestation of this ill-will was clear at the meetings of The White Council. Gandalf had mentioned he had suspicions that Saruman was being corrupted when The White Council, due to Saruman's leadership, did not immediately attack Dol Guldur and Gandalf went by himself to learn the true identity of The Necromancer. There are other manifestations, the above quote indicates Saruman's ill-will was hidden for only a short time before he learned that Cidran thought Gandalf was more deserving of Narya.
So why would Gandalf have said that Saruman was very wise and go to see him for counsel about The Ring of Power if he knew that Saruman was already looking for it for himself?