Setting
Rural Kansas
The Wizard of Oz famously breaks its story down into two sections: Kansas prairie, which is shot in monochrome brown, and Oz, which apparently banished from its shores any color that doesn't make your head spin. That's not an accident, and the rest of the set design works to push those differences.
As we talked about in the "Symbols" section, we first get the sense of how incredibly not-interesting Kansas is. The shot we get of the prairie shows it stretching off into nothing, without so much as a grain silo to perk things up. Uncle Henry's farm is no-nonsense and by the numbers, lacking any distinguishing features to make you say anything more than "blah." One of the reasons why the cyclone—the bridge between Kansas and Oz—grabs our eye so sharply is because nothing else in Kansas merits any attention at all.
That's in the beginning at least. At the end, the filmmakers want Kansas to feel like that snuggly blanket you grew up with and now only get to use at Christmas. Notice that we don't go anywhere but Dorothy's room when we get home: no reminders about how flat the landscape is or about how there's nothing else to do. Dorothy's in her bed with all the people she loves around her, and those dull colors mean that nothing scary's going to jump out and get her. Those same bland qualities now soothe and relax us, as we see what Dorothy's been fighting for the whole time, and we want her to be happy and safe now that she's found it again.
Oz
Oz, on the other hand, needs to surprise us at every turn. Most of those surprises are delightful, or at least have a snappy sing-along to accompany them. But some of them are of the nasty variety. That gives them a certain potency, since we never know what's around the corner.
The Technicolor plays some part in giving us that, but it goes beyond just pretty colors. The painted landscapes on the set are designed to look just a little surreal, with hills rounder than they would be in real life, and trees sharper or pointier. There's a lot of tropical birds like toucans and emus hanging out around the Tin Man's house, even though the trees look like classic European fairy-tale types. The trees all have expressionistic faces (the ones in the haunted forest still freak us out). And if you look closely, you can see the cool ways the filmmakers blend the painted backgrounds with the actual props and sets. It brings that surreal energy to life in ways you just don't see anymore…again, in contrast to Kansas, which sticks to garden-variety photorealism as much as it can.
The beauty of Oz is intended to show Dorothy how wonderful the world can be sometimes, but also to stress that home is home, no matter what. The surrealism always holds a hint of danger, as with the poppies that put you to sleep. The slightly skewed visual look reminds you that as awesome as it can be, Oz is never entirely safe. It's a great place to visit, but living there? That might not be so grand. It takes some very good set design to perform a switcheroo like that, but as we've noticed, The Wizard of Oz wasn't exactly hurting for brilliant filmmaking.