I’ll try to pass along some workflow ideas, but they change and evolve as I learn. There is a choice you have to make at some point in the processing. It can be made initially or later. You have to decide just what color you want the sky to be. The color of the night sky is a fun and interesting discussion because there are major differences in what we see with our eyes and what the camera sees. Also, the color or color temperature of the sky may change at different times during the night, and there can be different colors in different directions at the same time. Making maters even more complicated is the lack of agreement on color temperature we use in our cameras. I now use a manual color temperature of 4000 degrees K. If you want the sky to be blue with a cooler look overall, you will want to process the sky with a cooler white balance. This usually turns the highlights in the Milky Way more of a magenta color. This is a relatively common choice photographers make. The sky experts say that the real color of the Milky Way is more yellow, and the color of the sky is warmer color. The problem is that we do not actually see the sky as a warm color. I aim for a compromise. So you decide what color you want the background sky to be. This method is flexible enough to tailor to your wishes.
If you use the Temp and Tone curves (in LR or PS RAW) to make the darkest part of the sky a dark grey, several things happen. The Milky way develops a yellow hue rather than magenta. The stars develop individual colors. If you look closely you will see blue, yellow, red, white, and occasionally orange stars, with the largest number being blue. This reflects the temperature at which the stars burn, the bluer stars burning hotter. This is a better reflection of real life. If you process the sky as a “cool” blue color from the start it will in part cover up airglow (green) and light pollution (yellow, orange, red). If you process the sky more neutral or more “warm” you will find a lot of subtle color coming out. The airglow and light pollution shows up much more vividly. It can be attractive or a nuisance. You can then try to make the light pollution and airglow go away, or you can embrace it and work with it. Light pollution can make the sky look very surrealistic, with lots of green, yellow, orange and red. It can look beautiful but very otherworldly. Lots of photographers try to get rid of this color to make the sky look more natural or more like we expect. Lately more have embraced the airglow. I have stopped trying to get rid of these colors and have embraced them (at least for now). A good choice might is to process the sky more neutral to start with and then twerk the color later as desired, as described in detail below.
The initial part of processing can be done in Adobe RAW or Lightroom (I use Lightroom). For processing I usually make the darkest part of the sky as neutral as possible in Lightroom (using the Temp and Tint sliders). It is hard to get it perfect, at least for me. When I say I make the sky as neutral as possible, I do this: I pick the darkest area (usually near the upper corners) and try to make it grey or only slightly tinted. The easiest way to do this is to look at the separate histograms for red, green and blue in the histogram graph and use the Temp and Tint sliders to get the separate color curves to overlap or match up as best you can. Overlap or superimpose the blue and yellow peaks (in the part on the histogram that represents the sky - the large rightmost peak), then superimpose the green and magenta peaks as best you can, then go back and superimpose the blue and yellow peaks again, keeping an eye on the darkest sky. This means that you no longer have a cool or warm tone bias. The more neutral you make the dark sky, the more subtle color you can bring out elsewhere. The stars develop individual colors. If I want to end up with a blue sky, then you can do it later in PS with Much more control. Do NOT make the sky blue at this early step. To get a preview of what the sky will eventually look like go to the “Tone Curve” function in Lightroom. Click on the little square in the bottom right hand corner, so you can manipulate the curve freely. The place a very steep “S” shape on the curve, up at the top and down on the bottom. This will increase the contrast tremendously, and you can see which way the color is trending. If you cannot get it completely neutral or grey then i leave a minimally blue bias in the darkest sky. Then right click on the curve and click “flatten curve”, and the sky will return to a grey flat color (contrast will be applied later in Photoshop). Usually the upper sky is so much darker than the lower sky that I may lighten the upper sky with a gradient in Lightroom to make the sky more uniform in density. This helps a lot in processing the sky later. I also do noise reduction, sharpening, and lens correction in Lightroom and export to Photoshop.
You can do all of this in Adobe RAW with the exception of the gradient function. In Lightroom or Adobe Raw here are some good initial settings for sharpening and noise reduction for an ISO of 6400 to 12,800 for a Canon 6D or a Nikon 810A:
For sharpening, Amount 50, Radius 1.0, Detail 15-20, Masking 75. For noise reduction, Luminance Noise Reduction about 50 with Detail 50, and Contrast 0. Color Noise Reduction about 17-20, Detail 50, smoothness 100. The amount of noise reduction you need will vary with your camera and ISO. Do NOT increase Contrast at this stage. Do not increase Clarity at this stage. Do NOT increase saturation at this stage. You can do them later in PS with more control. I then export or open in Photoshop. To do this right click the image and choose “Edit in Photoshop”. In Photoshop I carefully select or mask the sky and foreground and place them in different layers.
In the Sky Layer and may increase the vibrance a slight amount (10). Now you want to increase contrast and darken the sky. You can darken the sky primarily in Curves by adding contrast, but if this is your primary way to darken the sky, then you may be adding a lot of noise and graininess. I personally AVOID the Curves function. I find it changes the colors and contrast and they quickly get out of control. I see many photos on the internet where this has occurred. I mildly increase the vibrance first, as stated above, and then mildly darken the sky by using the Levels Command. I simply take the slider on the far left and move it to the right. This darkens the darks and adds contrast, but does not change the quality of the color as much as "Curves". I find that this will really bring out the subtle sky colors if you start with a neutral sky. If you make the sky very blue in Lightroom from the very beginning, them you cover up a lot of the subtle colors in the sky, and also you can give the airglow and light pollution unpleasant color casts. I usually lighten the sky in a band immediately adjacent to the horizon. This area frequently gets to dark later. With the sky mildly to moderately darkened I may use the Color Balance function to slightly add a little blue to the sky. It does not take much! You can do this to our personal taste. I frequently choose the Milky Way with the lasso tool and feather it about 250 pixels, and then use then adjust the brightness and contrast. I may apply a mild "S" curve in "Curves". I then do "Select" and "Inverse" to choose the sky outside the Milky Way and use the Levels Command to darken the sky further by moving the left (shadows) slider to the right. Airglow, typically in the lower sky looks better if you lighten that area to some degree. It is now basically done. You can do tweaks for specific parts of the sky, and many times I do some light dodging and burning to fine tune areas and lighten the area around the Milky Way. You can make minor adjustments to tweak the color to your likes. You can notice that the stars and Milky Way still retain their color and you have not "bleached" the color out of them.
I then chose the foreground layer and adjust the contrast separately in curves. It does not take much! If you start with a neutral sky, then the color of the foreground is usually pretty true. You may need minor adjustments in color. I do a little dodging and burning for hot areas or areas that are too dark, and that’s about it. I commonly increase the local contrast using the Unsharp Mask function, using an Amount of 10-20, a radius of 50-60, and a threshold of 0. This is a different kind of sharpening that relies more on local contrast than on edge sharpening. This introduces less noise. This increases contrast and gives the impression of sharpness, without increasing the perceived noise too much. If there is too much noise I use the Topaz noise reduction plug-in. It is remarkable good at reducing noise without making the image too soft.
I discuss noise problems in Nightscapes in a different article.