This photo which was taken by a friend in the French Alps shows beautifully how we can use two blues straight from the palette, to create a stunning sky.
Can you see them? There's ultramarine blue at the top, and cerulean blue at the bottom of the sky.
What these colours do is to stop the sky being flat. If you go outside and look at the sky you will see that it is usually paler towards the bottom, as the dust in the atmosphere weakens the colour. By painting with these two blues we create the sense of the sky curving above us and receding into the horizon. Try it and you'll see!