Sky color at night in a city is affected by both timing and camera settings. If you want a richer sky, try shooting during blue hour (just after sunset) rather than when the sky has gone fully black. This often gives a more attractive result and better balance with illuminated buildings.

For capture, meter for the sky first, lock the exposure, then recompose. That helps prevent the sky from washing out or shifting unpredictably.

White balance is also a big factor, especially under city lighting. Try a different WB setting such as Cloudy, or better yet shoot RAW so you can fine-tune white balance afterward. RAW gives you much more flexibility to correct blue, black, or orange/polluted-looking casts in post.

If adjusting the sky makes the foreground look wrong, create a second edit or virtual copy with a different white balance for the sky and blend the two in Photoshop.

In short: shoot at blue hour, expose for the sky, and use RAW/white-balance adjustments to refine the color.