In May's House Beautiful, Houston Interior Designer Eleanor Cummings recommends Sherwin Williams Rarified Air (SW6525) as her go-to 'tranquil' color.
I did a double-take after seeing the HB images of the color; they are stunningly beautiful - a pale, pale, gray - but the Sherwin Williams on-line color sample looks very blue. Not entirely a bad thing - just know going in that Rarified Air has major blue undertones. Eleanor says that Rarified Air is "not that sweet blue, but a sophisticated blue."
Still, I have to think that the lady who puts these rooms together has to know what she's talking about:
photos from Eleanor Cummings Interior Design