Actually, I looked it up and it says the sequences in Kansas were shot in black and white, but the sequences in Oz were shot in technicolor, and that is how it remains today.
Actually, it was sepia, not black and white (yeah, I know… picky, picky).
One of the great moments in the film is right after the house lands in Oz. The film is still in sepia until she opens the door. Anything outside the door is in color while the rest of the house is still colored in sepia.
If you’re curious as to how they did that in the late 1930s, the scene was shot in color. The set (including the dress that Dorothy was wearing) was sepia-colored.
The Wolf