I had assumed that the coloration in the sky was intentional. First of all, it's definitely there. I checked the image on my monitors at home and at the studio, both of which were custom profiled with an X-Rite colorimeter. It's there. The reason it's there is because of the way you toned it. My personal work is almost exclusively B&W and I always tone my images to simulate traditional selenium toning in a wet darkroom. The best way to accomplish that in Photoshop is not to evenly tone the entire image, but rather to use the Photo Filter, which is in the Image/Adjustments menu. By checking the Preserve Luminosity box in Photo Filter, Photoshop will apply toning differentially, rather than evenly across the entire image. It will apply more toning to the dark and mid values and progressively less toning into the high values. This is a much better approximation of traditional wet toning and it avoids colorizing things like clouds where you don't want to see it. The fact that you can't see it means that either your monitor isn't calibrated correctly or its color gamut is too limited, or both. Mac users often assume that simply relying on the built-in eyeball calibration routine in OS X's Display preferences will be enough to ensure that their monitor is color accurate. Trust me, it isn't. I found that out the hard way. The only way to calibrate a monitor accurately is by using a good colorimeter.
I do use a colorimeter, a ColorVision Spyder3 Pro, always connected to my PC and always self calibrating, so I can only assume it must be an artefact of the low quality JPG compression I use for posting on this forum - in fact I would bet that is exactly what it is.
The image on my computer is a 125mb Prophoto 16bit uncompressed Tiff and the version posted on this forum is a 128k Jpg, so the image you are looking at, has been compressed and reduced in size by 1,000.
Anyways, here is a dead straight desaturated mono version, so any mysterious colouration artefacts anyone now sees in this version, are definitely not there.