What color is the night sky? It's not that simple a question. The night sky is no more black than the day is blue. Clouds, moon and man-made light all have an impact on the color of the night sky. While black will certainly be part of the mix, for best results, you'll want to add some blues and / or grays.
Painting the Sky
Like a daytime sky, there will be variations in color and hue from one part of the sky to another. If you plan to paint any details along the horizon, you'll need to make the sky a little brighter to make the contrast. Black on black doesn't show very well.
The easiest way to accomplish this color gradient is to mix the paints directly on the backdrop. Start with your darker color (a navy blue, dark gray or even black) at the top and a lighter version of that color at the horizon. You can darken any color by adding a little black. It is usually easier to add black to a color to darken it than the other way around.
Next, pull the light color up from the horizon and pull the darker color down, blending the two as you go. Work quickly while both colors are still wet and you'll get a nice natural transition. Oil paints will give you a longer drying time, but if you work quickly you can easily get the job done with acrylics too. Acrylics will dry completely much faster allowing you to judge your work and move on to the next step much faster.
You can add clouds after the sky colors have dried or while the paint is still wet. For more defined clouds, allow the sky to dry. For a more moody, softer look, apply the cloud color while the paint is still wet, blending it into the sky colors along the edges.
There's nothing like a starry night sky. There are two ways to add stars. Using white paint, you can apply stars individually using a very small brush or a pin. For mass production, load up a larger brush and flick the paint onto the backdrop. Practice this technique on another surface first until you are comfortable with the amount of paint, force and distance needed. You'll need to stand a good distance from the backdrop to prevent large splatters. Protect the layout and room with plastic sheets. Depending on the amount of stars desired, you may find its still faster to add them individually when considering prep and cleanup time.
If you've used acrylics, your sky will be dry to the touch within an hour of painting and completely dry within about 24 hours. Oil paints may take as long as three or four days to dry completely depending on how thick you've applied them. Once the paint is dry, if you are not happy with the results, you can paint another coat on top. If you are satisfied, it's time to add some details to the horizon.