Thanks to everyone.
I still think that all skies presented in this thread, including my own post, are way too blue. Go and have a look for yourself, especially if you live in a Nordic country. The sky should look much more like the sky below (the whole photo has been subjected to the automatic neutralize color option in the color balance module to remove a blue cast).
I live in a high, dry place, and blue skies like that are common. From a fishing trip a few years ago, about 9500ft MSL:
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Interesting.
If I look outside right now (20:43 CEST in the Netherlands) I see a sky that is about as blue as the lower parts of the sky in my example, your latest shot doesn’t have any (or very, very little) blue in the sky at all…
Your first shot was taken at noon (assuming the timestamp is correct) and remembering from my vacations in Denmark and Sweden I do seem to remember that the sky can be pretty blue with nice white clouds.
First thing that comes to mind: Is your monitor calibrated correctly? And do you see this with all images (not just your own but edits by others).
When sailing in early summer in gothenburg last year on sunny days the sky was very blue - only on cloudy days or misty times it was more greyish. At least over water there’s a good chance to have a even a little bit of mist turning the sky into a light grey.
So i‘m pretty sure your nikon is as ok as my canon is 
Another one, shot with a lens with 72mm filter thread, where I clearly don’t have a polarizer to screw in (default processing in RT):
For what it’s worth, the Colorchecker Passport has a square of what is considered to be sky blue by some autonomous group of chaps. I believe it’s the 3rd from the left of the top row?
Sky blue in germany, sweden, south of france… are completey different things imho
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first photo : exposure compensation 1.67EV, tone curve. The sky is not natural as I imagine it was light blue. But due to the luminance of main subject, the sky is underexposed and can get a bad color.
The only way I see, when it happens to me, is to correct sky color with a mask. I made that with GIMP and now with ART.
This second interpretation looks like scene is located in a southern country
Sky blue can be quite different but location is not the main factor. The angle of the sunlight, clouds and atmospheric impurities are important.
And also the human eye (brain) can adapt to very high brightness and still recognize colors. This is difficult to transfer to photo paper or the screen.
I shoot a wide angle lens a lot, a 24mm prime on a full frame body, and the sky is often dark like this. I like the look, so I keep it in a lot of my photos.
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I grew up in western Canada, and the skies there were very blue, maybe just a hair lighter than in @ggbutcher’s example. For most of my time in Toronto, only the center of the sky has been a strong blue; with the reduction in traffic caused by the pandemic, the blue has gotten more prominent and widespread. I remember seeing works by Dutch painters and thinking, “the skies are not realistic, they are too iight, too pastel.” Then I visited the Netherlands, and discovered they were spot on.
There are so many factors including weather patterns, turbulence, temperature, atmospheric composition, time of day, angle of sun and reflections, humidity, camera gear and settings, air pollution or smog, magnetic interference, colour management, chroma or saturation manipulation, eyesight, cognitive patterns, cognitive dissonance, allergies, trauma, someone farted or talked behind your back, etc.
Thanks to everyone.
Lots of opinions on the color of the sky. I can assure you that the sky can be blue in Denmark, but the color is very much depending on the angle to the sun and other factors.
My original problem/question falls in two parts:
How to change (edit) the color of the sky (maybe change it to almost white) using darktable preferably without a mask and avoiding halos at the border between the sky and other objects or changing the color of all other objects as well.
Having read all answers, I conclude that without a mask only relatively limited color changes are possible in Darktable.
Now for the other part:
The Iphone shot looks unrealistic. The image has not been edited in any way apart from the Iphones build in jpg conversion, so the result is what the Apple engineers think that the customer likes the most.
Reading magazines (photo, nature, wildlife etc.) we all get accustomed to colourful, sharp and high contrast images. The sky is often very dark blue. It looks amazing, but the images are not realistic. I admit that I tend to edit my photos to achieve this kind of look as well.
Is this look only the result of editing (in-camera jpg or post production) or is the dark blue look also build into the hardware of the camera i.e. the sensor?
I am not sure these all can be done all at once…
You can try to reduce the saturation of the blue colour and raise its luminosity. The are more modules that can do this in darktable, so try and see which Briggs you the result you want.
Just worked on the sky, except for a little work on the statues to touch up what my other work changed. @obe: Are you saying the sky really was nearly white, but the camera produced dark blue?
DSC_2122_06.NEF Bill M .xmp (21.7 KB)
Edit: Filmic V4 version
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Yes I do. I think that my camera has a tendency to produce skies too dark blue. A pale blue sky is how I remember the scene. More like the sky in the photo below……
I am going to be picky, but if you use raw, what is out put by camera are the data as produced by the sensor in the camera space. Only when a camera profile is applied that we can speak of colors (blue…).
So it may happen that the camera profile you use is not good for those landscape photo. The first step seems to me to be sure the camera input profile is ok.
Regarding color of sky, in your photo, the sky is largely underexposed due to foreground very luminous white subject and will necessarily appears dark blue without specific correction.
I should add that the thickness of athmosphere passed through by light is very important : sky is darkening with altitude;
there is a very strong gradient from vertical sightseing to horizontal.
I shot the photo in raw and used the standard color matrix supplied by darktable. Scrolling down the available profiles no one seems to produce a better result.
It’s correct that the photo is underexposed (to avoid clipping the white figures) but adjusting the exposure in darktable should compensate for that and bring back the correct blue in the sky.
When I look at the pictures shot with my wife’s Iphone – no editing outside the camera – the skies are much more dark blue than in real life. It looks nice, colourful and dramatic but it is not realistic. We have become used to colourful, highly contrasted and very sharp images. Real life has become dull…
The actual color of the sky depends heavily on time of day and the atmospheric composition at your location at the time of the photograph. See e.g. here for Granada, Spain, here for Ontario, Canada and here for New Delhi, India. The presence of scattering particles in either upper or lower atmosphere, and the presence of ozone can also influence the color spectrum.
So blue, isn’t blue, isn’t blue
Though I agree that the iPhone doesn’t seem to aim for accurate color reproduction…