[Kaiserin]

I was thinking that "Gandalf the Grey" was probably a title others had attached to the wizard because of his gray cloak, then he began calling himself that when he was known as "the Grey". Elves called him "Mithrandir", which means "the grey pilgrim"; a different people from different areas called him other things...

The istari's robe colors also said something about their "areas of expertise" - or something like that. Radagast the Brown was more involved with flora and fauna and earth-stuff, and not that much involved with affairs of the Ring like Gandalf the Grey and Saruman the White were.
When Saruman became corrupted by the power of the Ring, he actually ceased to be "white". He wore a robe that merely appeared white but up close it was actually made up of many different colors.

(If I just said something incorrect, anybody feel free to comment! [img]smilies/biggrin.gif[/img] )