Saruman was a turn cloak. I don't think Radagast was; the worst you could say of him would be that he kept his cloak in the wardrobe and only brought it out for special occasions - Christmas, funerals and weddings as it were.
He didn't turn bad, and that's the point of the topic. If you were to make a list of good guys and bad guys you'd probably put him in the good box. Even if be is a little scatter brained or misguided.
I wonder if Gandalf should have taken him under his wing as a sort of cousin and they could have travelled Middle Earth together, like the blues. Gandalf keeping Radagast on the path and Radagast lightening Gandalf's burdain. Who knows?