To paint the sky of this moonlight seascape painting, I started with violet and white, and slowly started mixing little ultramarine blue. But the focus is to keep it more towards violet. Keep the sky dark towards the top and as you approach the horizon, start mixing more white to reflect the lit up sky from the moon. Also keep adding some bold strokes of white showing through that will act as the clouds. So instead of blending it all together, leave some of the brushstrokes as it is. Note: I added a touch of burnt sienna with the other 3 colors, to desaturate the colors a bit.. but if you are someone who loves the bright colors, you need not do it!