What color should I paint the walls in my bedroom?

My husband and I just bought a new comforter set in slate gray. I am looking for a color that is soothing and will encourage sleep that will go well with my new comforter to paint the walls. I have been looking at shades of blue but am torn between that and maybe an indigo or purple color. Any suggestions would be greatly appreciated. I am willing to consider other options of wall color though. Thanks!