Black & Gray tattoos
http://foundationmedix.com/blog-2/ Typically, black-and-gray tattoo work is produced by diluting the black ink with distilled water in varying proportions to create a “wash” that results in lighter shades. http://rpstransit.com/resources/
Gray shades can also be produced by mixing small amounts of black ink with white ink, which produces a thicker but brighter result and requires a slower application.
Shading is typically an important component for these types of tattoos as they will fade over a period of years without strong black tones, which provide contrast and allows the tattoo to stand out.
Subtle kinds of shading in black-and-gray are considered to require a high level of skill and illustrates professionalism in the industry.