If you’ve ever walked some of the indoor malls in Vegas, especially Cesar’s, you can see well done sky murals- all one color blue with clouds painted over top. You can get this same look and feel. (OK ok, maybe I’m dating myself- because I’ve not walked THAT particular mall in over a decade and not sure if its there).
When developing a mural intended for a ceiling, its certainly tempting to grab the freest sky image you can from any of the image sites, or, possibly cheaper, just take a picture yourself. The problem with this method is the variation of the sky color from east to west, or north to south.
While this may not seem to be a ‘big deal’ it can make your ceiling mural look washed out, and, depending on the lighting, very unnatural. It is my suggestion to either find the most atonal image possible for a sky, or replace the blue sky component altogether. Depending on the amount of time you want to spend on it, and the desired clouds, you output/color/whispiness quality can vary.
Take one from the opulence of Cesar, and start with a fixed blue sky, and go from there.