The Cool Grays have a distinct cool soft Blue undertone to them, giving off a clean natural feeling. They can be mixed into cool colors to neutralize them, making other parts of the tattoo stand out more. use them as though they are a Deep, Medium and Bright trio.
They can also be used on their own to create cool Gray shadows and forms. Basically, use them as a desaturated Blue, which can be mixed into analogous color groups near the Blue part of the color wheel.