Okay, without getting too technical about the way the earth's blue sky has various shades of blue----which color blue did you choose for a background on your model railroad?
Which brand & store? hamr
Which brand & store? hamr
Shade is less important than variation... get a good deep blue for your sky, and a quart of white. Mix up a lighter shade of blue using the white and your blue, and paint the bottom 1/3 of your backdrop with the lighter shade, and paint the top 1/3 your original dark blue. Then blend the light and dark together while they're still wet. After they're dry add clouds, and viola! Sky!
Shade is less important than variation