Skimmed the above; just any decent sky-blue (and we all know that the sky is many colors of blue, at many times) will be what you want. I did a medium sky blue then used cheap flat white spray cans paint to spray some "mid-summer" soft, hazy clouds. Worked well,
I think. Easy. Needs maintenance. On my layout it is eternally Noon, sunny/hazy, July 1, 1950.
Overly "painterly" backgrounds (especially those done with oils) more often that not look
like cartoons, despite the best of intensions. A backdrop should not be noticed; if it draws your eye, it is too much. Good photo-rendition backdrops can get away with more presence, if properly handled and "perspectived" (now a word!).
Lightning, thunder, etc - ah, no.
