[Mithadan]

Never really thought about it before but here's my take on your topic:

White - self-explanatory. Pure and the opposite of black.

Grey - white soiled by his labours, but white nonetheless.

Brown - dedicated to the earth and the things which grow and run wild on it.

Blue - no real idea. Tolkien later wrote that the Blue Wizards may have set up cults of some sort in the East to oppose Sauron and thus were not wholely unsuccessful. So perhaps blue means royalty as in the leader of such a cult?

Many-coloured - easy. "He who breaks something...has departed the path of wisdom". Many-coloured is broken white.