Gandalf says, "Yet I had caught no hint of anything wrong in his voice or eye at our meeting. If I had, I should never have gone to Isengard, or I should have gone more warily. So Saruman guessed, and he had concealed his mind and deceived his messenger. It would have been useless in any case to try and win over the honest Radagast to treachery. He sought me in good faith, and so persuaded me."
This leads me to speculate that Radagast never met Saruman in Isengard. Isengard was full of dubious factories and Uruk-hai by this point. If Radagast had seen Isengard as it now is. surely he would have had doubts and suspicions which would have been apparent to Gandalf.
I guess that however Saruman communicated to Radagast, it was not at Isengard. So, although I like the thought of Radagast being called to Orthanc as a consultant on hues, (Saruman - "Dear Radagast, I have become bored with the color 'White'. I am looking to re-brand myself with some more modern and vibrant hue. What would you recommend?" Radagast- "Well, Saruman, as I'm sure you know, white is just all the color spectrums combined. Why not make that explicit and call yourself 'Saruman the Rainbow'?" Saruman - "I like your thought Radagast, but that rainbow image is altogether too trendy, and, I feel, appealing mainly to a demographic of pre-teen girls who like unicorns. However, 'Saruman of Many Colors' has a certain ring to it!"), I have my doubts whether Saruman and Radagast ever met in Isengard for Saruman to give the 'seek Gandalf' errand to Radagast.
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I suspect communication from Saruman to Radagast in some other place, or by some means other than face to face conference.