Whether you are picking a sky blue for your porch ceiling or for a cloud effect in a nursery, a light blue ceiling is appropriate in any room. Paired with any wall color a sky-blue ceiling has a cool, calming effect, but only if you choose the correct shade of paint for the look you want -- vivid, sunny day or cloud-swept, ethereal evening.
Scan the paint chip display at your local store, the first thing you notice is that colors are grouped by clarity. There is a section of grayed colors, a section of muddy colors, clear, and bright colors. Mentally discard the grayest section along with the shockingly bright section. A good sky blue has a clarity of color that isn't muddled by gray. Don't be tempted by the shockingly bright section. While they appear to be clear blues, they are way too intense when painted over a large surface, like a ceiling, and are too shocking for most applications.
Focus on color family. As you look at all the paint colors in the display, you see that greens fade to blue which then fade to purple. Choose a blue that is in the middle of this range. This technique will yield a color that is neither green or purple. Because of the nature of a sky, if you have difficulty finding the true center of this gradient, err on the side or purple. A greenish blue is much less flattering than a blue that feels a touch too purple.
Now that you have the correct chip, hone in on a specific color If you notice your paint chip has colors from pale blue to a dark blue. Some paint companies' chips only go from pale to medium blue. Mentally discard the very palest color from consideration. On the ceiling shade just tinted blue it isn't bright enough to convey the feeling of sky. Rather, it more resembles an off-white that can be unsettling. The next color on your chip, while still light, but tending a touch toward a medium blue is a great bet.