nice captures. The colors are really vivid! Have you postprocessed the colors or are they out of cam?
As you requested some feedback, here you go:
Personally those colors don't look very natural to me, but I can't compare it to the real colors, so that's just a personal impression.
Regarding the composition of the images I would either try to get a bit structure to the foreground to see how the lights of the sky lightened the city or another option would be to use the shape of the city to get the images more interesting.
To get more structure to the image I would suppose to use RAW file format with a camera that has a high dynamic range and push the lights and shadows in the postprocessing or use HDR to capture the high dynamic range (bright sky, dark city).
To get the shape of the city you might need to underexpose the images more to get it darker/completely black. You might also want to remove the lights (like bright lamps or windows) in the city, because they are a bit distracting, but that needs some time in postprocessing. You can also use a bit more space for those shapes IMHO.
For this images above you have chosen option two and that's a really good start. The sky has some nice structure and colors. You might want to remove the lights in the city, because they are a bit distracting. You can also use a bit more space for those shapes.
Hope this helps! What is your opinion on that points I made?
PS: I am not a native English speaker, so I hope this is understandable.
The colors are an attempt to capture with the camera what I was seeing with my eye, and the deepening of the color is due to liberal use of exposure compensation. The camera still didn't capture some of the mid-tones that I was seeing.
Your comments are extremely useful, and I will attempt to put them to use. And your English . . . I would not have guessed that you are not a native speaker.
That's a good start, I like keeping things natural. Anyway, your camera might have applied some additional things.
The lack of mid-tones could be due to the sensors low dynamic range. That's pretty hard for every camera to capture what our eye can see.
PS: I can't really believe that part about my English.