Gray is already known as a serious hue, and cool gray doubles down on that level of seriousness through the addition of a tinge of blue. Cool gray is a mixture of black, white, and a hint of blue.
Timeless and steady, cool gray can make your design soothing. It is also used in literature to describe the fog that arises from cities near bodies of water.
A practical color, cool gray makes an excellent background since it highlights other colors around it.