Heavy Howitzer Hauler

Taken at the local armoury with a 5D, 24-70 and GND. I think the upper part of the sky looks a little odd. Any good hints on how and if this image can be helped at all?

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It looks like you just generally darkened it. The best way I've found to do that is to select the area you want to apply an effect to use a layer Mask to confine changes to just that area.
Advantage is that the mask is reversible.
Selecting areas in PS.

Need to remove a background from something?

Thanks for the hints. I have tried a couple of times this shot without the GND and the track always ends up too dark and drab. The GND really helped get the track the proper contrast, just wish the upper sky was not so vivid.