To produce the image below, I first took advantage of the symmetry of the render to effectively double the number of samples, using Photoshop to duplicate the high-dynamic range EXR image, flip it horizontally, and composite it onto the original. Then I used Photomatix to tone-map the processed image. The solar (top) side is much brighter than the anti-solar (bottom) side, so without tone-mapping it wouldn't be possible to expose the entire sky nicely at one time—part of the image would be overexposed or part would be underexposed. The tone-mapped image is over-saturated and the relative intensities of different parts of the sky are not accurate, but you can see the entire sky and you can see the colors clearly.
And below is the PNG that came directly out of my renderer. I applied a mild S-shaped transfer curve before writing to PNG.
|Twilight, directly from my renderer.|
The sky was clear this evening in Philadelphia and I was able to see the Earth shadow. The colors in the real sky closely resembled those in this render.