Still on the issue of the colours of the
Istari:
I think the colour white has directly to do with the so-called 'Valinorean stature' of the
Istar in question;
Saruman was the most powerful of the Istari, as designated in Unfinished
Tales and by his leadership of the Istari in LotR, and wore white. When
Gandalf became the more powerful of the two, he wore white; and at this time Saruman had donned the multicoloured robes and basically resigned from his position as head Istar.
The way I see it, the lighter colour an Istar's robes are the more powerful he seems to be (not because of the robes, they just designate the person's stature): Saruman was White, and was the most powerful; Gandalf had great skill with fire, and accomplished the most of all the Istari because of his psychologicla skill and his kindness, and was thus only less powerful than Saruman by the definition of 'Valinorean stature' (which, as I see it, basically denotes magical prowess as well as wisdom), and Gadnalf wore Grey which was a slightly darker shade of white basically;
Radagast had no real skills, it seems, except that he was able to converse with
animals and probably
plants as well, but who is to say that all
Maiar couldn't do the same? Anyway, he accomplished least of the Istari, though he did not wholly relinquish his quest, and Radagast wore Brown, which is a dark colour in relation to White and Grey.
And as for the
Ithryn Luin, as their colour is Blue it would seem to denote that they were less powerful than Saruman or Gandalf but still possessed some sort of magical skill.
This is my theory anyway.