This transformation is designed to fix up the green cast in the sky in the image near the horizon. I played with a couple of ways to do this, but the simplest was to use the color curves transformation, use the color picker tool to find where the blue part of the sky at the top of the image is, and lock that down in the curve (the right hand vertical connector), then use the color picker find the greener part of the sky nearer the horizon, and push that toward the bluer part. The resultant image is then used for the next transform, to clean up colors in the clouds.