When I saw Oh Brother in the theater, the colors were bizarre to say the least, but it was the intended effect. As I recall, some of the color was obtained in post production, but in some cases they actually painted the foliage to get the yellows instead of greens. On the big screen it was striking, incredibly yellow, and you could actually hear the audience reacting in some scenes.
IOW, it didn't look real, and wasn't supposed to.
Here's hoping the BD recreates that effect.
Edited to add:
Watch the "Painting with Pixels" feature on the DVD. It has several before and after shots of the color shifts. (which are also very washed out on the DVD where they were not in the actual film) Also important to note that the original film negative is useless for a new transfer. The transfer would have to come from a later film transfer from the digital elements. Unless they went back to the actual digital elements for this release.