"Australian Sky Blue is a simple blend of Ultramarine and Titanium White It is very convenient to have this mixture in a tube as the sky forms such a large portion of many landscape paintings and this colour is a great starting point for making sky colours. It would be a mistake in most instances to just simply paint an entire sky using the blue from this tube. Sky colours vary a great deal from day to day and even from hour to hour. The sky overhead is often a deeper colour than this but the colour changes as you look closer to the horizon and near the horizon the sky is often a lighter and softer eggshell blue. No single colour can depict all these shades and Australian Sky Blue does not pretend to do so, but it does do what an ideal sky blue colour needs to do which is provide the base from which to create the variety of sky colours found on the average day.
There never has been a true sky blue single pigment. Azurite, used from ancient times until the 19th century was closest for a long time, but it still needed to be lightened with white to be useful. Cerulean Blue is the modern equivalent of Azurite and when mixed with white can be great for painting the skies of northern Europe. The Australian sky, however has a very different quality and mixtures with Ultramarine Blue are often closer to the sorts of colours in an Australian sky, especially in the summer time, or in the arid regions. Thus Australian Sky Blue is just right as the starting point for most Australian sky colours.
Because the colour varies so much from overhead to the horizon it can be a very good idea to use Open Medium MM31when painting the sky. Open Medium keeps the paint wet for longer, depending on how much is mixed into the colour. This enables wet-in-wet techniques and makes blending of the sky colours very easy. Because every sky is different the exact colours chosen will vary from day to day. Ultramarine Blue, Mineral Blue, Primary Blue, Cerulean Blue, and Cobalt Blue will all be excellent choices as darker mixing colours depending on the quality of the light on any given day. To get the soft eggshell effects seen near the horizon try a little Australian Ghost Gum or Antique White. Just a little will soften the colour nicely. Working wet-in-wet will facilitate gentle graduations of colour that will be pleasing.