Instead of muted colors, there's a very bright orange, and it's a natural setting (with birds flying across the sky, even), rather than the man-made environments seen previously. The stark contrast between this single shot and everything that comes before it visually illustrates THX's breaking free from a very regulated society.
I also noticed that the soundtrack for this scene is the first movement from Bach's St. Matthew Passion.
For what it's worth: years ago, I discovered that part of the Yardbirds' "Still I'm Sad" is used in the soundtrack near the beginning of Electronic Labyrinth THX 1138 4EB, George Lucas's student film that's the basis for THX 1138.