If you are dealing with a room with angled ceilings (like the one shown here) I always suggest painting the entire room the same color. Angles can disrupt energy flow and cause an imbalance in the space. Painting the ceilings and the walls the same color (in this case) assist in creating balance and harmony, making it feel whole.
In other words, you don’t want to draw attention to this type of ceiling. If the room is small and square with lower ceilings and you opt for a darker color on the walls, then I suggest painting the ceiling a lighter color which will lift the energy above you and suggest a higher ceiling. If the room is bigger and has a higher ceiling you might try painting the ceiling a darker color to (energetically) bring down. We humans like to get cozy and big rooms with high ceilings tend to feel more like museums and are a big off-putting for cozy living.
One last thing. Don’t be afraid to paint your ceilings the same color as your walls. Experiment and see how it feels. Remember you can always contact a professional color consultant or an Interior Designer who can assist you.