What colour should I use to colour in windows in a city night scene?

When it is dark the eyes can perceive fewer colours (look up rods and cones), so it tend to come down to very dark shade (black) and very light (white) with tints of grey in between.

Many people use dark blue and bright yellow to give a warmth and cool to the image.

Pale yellow

Depends. A good guide is that glass reflects whatever light is thrown at it.

Go outside and look.

Art is not made by committees, at least not good art.

You could use yellow.