Painting a moonlit sky requires some knowledge of how to create a sense of luminosity. The moon itself and the edges of clouds will appear to be lit up while surrounding areas will act to bring out this brightness. Every painting depends on contrast in colour and tonality, but this is especially true for paintings that convey a sense of luminosity. The light areas of the painting depend on the surrounding dark areas to set them off. Whether using watercolour or oil, your job as an artist is to balance these elements to achieve the greatest possible sense of luminosity.
Use a watercolour block or tape the edges or your paper to a board with masking tape.
Decide where to place the moon. Think of the possibilities: an upper or lower corner, or in the middle of the paper. Any placement is valid as long as you have considered all the options.
Leave the moon white and paint around it with a light yellow wash. Extend the wash to the edges of the paper, but create variations by making the yellow heavier in some places and lighter in others. You might even leave a tiny spec of untouched paper.
Paint using a rose colour. Work wet on wet, but not so wet that the paint pools up or spreads uncontrollably. As you build up layers of colour, try to let a hint of each underlying colour show through.
Continue this process using red, then a small amount of green, then violet and blue. The paper should stay moist but you might need to allow it to dry a little bit between layers so that it is not too wet. As you work, pay close attention to the balance between the light of the moon and the dark of the sky.
Soften the edge of the moon by going over it with a damp brush.
Allow the sky to dry to the point that it will not bleed into the moon. Apply very light washes of yellow and then green-blue to the moon. Use mostly water and very little paint so that the moon will retain its brightness.
Paint the entire canvas in variations of blue. This will be the sky. It can be very uniform or you can mix in hints of red, violet and even green. Mix in white to vary the value. The sky might be lighter at the bottom and gradually grow darker towards the top. Variations will make your painting more vibrant, but be subtle so that the sky remains a unified background for what comes next.
Paint clouds with a grey-blue that is slightly lighter in value than the sky. Soften the edges of the clouds. The clouds will gradually get darker the father they are from the moon.
Highlight the edges of the clouds that are closest to the moon. Use a white paint mixed with a little yellow. Blend the highlights into the clouds.
Paint the moon with the same yellow-white colour that you used for the highlights.
It you want to add trees, mountains or other landscape elements, they should be silhouetted and have very little detail. Use a very dark blue. Try ultramarine blue mixed with burnt umber.