It's a 'children's' book that I remember checking out of the library month after month, by an author with an alliterative name (Katherine Kooper? Catherine Cadwell?) about a society of muggles (I think this was the term, though it may have been something close that has been corrupted by Harry Potter) who live perfectly harmonious lives in a valley and wear all gray. But there are a few outcasts who like bright colors and... um... poetry? and go to live by themselves and save the people from mysterious mushroom invaders. It was along time ago, the details are hazy. There were two books in the setting, though I vaguely recall that the first was much better.
But I loved that book and I'd like to find it again! Any ideas?