I have taken a handful of classes. Most were for a specific task - binding, paper piecing, etc. One was a complete beginner class for a personal sized Rail Fence quilt. I felt I got exactly what I should have for each of these classes.
Consider this my shout-out for Julia at Little Blessings in Crossville, TN. She is an awesome teacher. I like that she didn't just say "Do it this way," but also explained WHY to do it this way. When you know the why its easier to remember the how, and you can make an informed decision if you decide to break the rule.
I agree with others who have stated that color theory should be a class of its own. I have a couple of books by Weeks Ringle and Bill Kerr, and they talk about color theory quite a bit. I would love to take some of their classes.