did you try breaking the images into mono RGB files? i think what happens is that in HDRComposition, only one mask is computed per exposure and the saturation point of r, g and b are different, so some over/underexposed stuff makes it in to one or more of the channels. in my case the red channel was correct, but green and blue had posterization/banding. but doing each channel individually, everything was mostly fine. there was a tiny bit of banding but it was easy to fix using the masks.
but it is a very nice image nevertheless!
rob
