[Noxomanus]

I believe each one of the Wizards had their own inclinations and specific task apart from the shared one. The UT mentions:

"It may be seen that they were free each to do what they could in this mission; that they were not commanded or supposed to act together as a small central body of power and wisdom; and that each had different powers and inclinations and were chosen by the Valar with this in mind." (UT,'The Istari',p. 510)

This could mean that what Radagast,Allatar and Pallando did,was part of a personal mission,specific to each of them. I can only suspect Yavanna wanted one of her Maiar to go as an Istar, perhaps to let the animals play part in the fall of Sauron or to make bridges between humanoids on one side and animals on the other. It's likely Radagast became too much centred on his personal mission and more or less neglected the common mission of the Istari.

Of Allatar and Pallando it's of course far more difficult to say anything about their personal mission. I do think that they,as Maiar of Oromë,were perhaps destined or commanded to go the East and stay there. I think the fact that Allatar took Pallando 'as a friend' and that he wasn't chosen by a Valar was the reason why both of them were clad in Blue----They came to share the personal mission appointed to the Blue and as such,had the same colour.

Gandalf was perhaps chosen for his humbleness and love of the Eldar (as well as his wisdom) and may have had the personal mission of ever-travelling wise man befriending all folks in time of need and in fact uniting them by their trust in him. Perhaps this was why he received a Ring. It would greatly help him with his personal mission,and Cirdan may have forseen it.

At last Saruman. His personal mission may have been to serve as the leader of the Istari. This may have been done because he was a Maia of Aulë,like Sauron was and he might be expected to be the one most able to understand and foresee Saurons actions as they were most similiar....more similiar in the end then was good for Curumo himself.