Tattoo Shading.
Tattoo shading is the part of the tattoo where a two dimensional tattoo becomes a three dimensional tattoo. It is the part of the tattoo where flat lines take shape and holds a certain degree of depth to make the tattoo "pop" out. There is even a whole style of tattoo shading now that focuses only on black and grey tattoos...no color, just black and white.
If a tattoo artist does not learn the technical yet critical aspect of tattoo shading, then a tattoo just falls flat. It is as simple as that. This one aspect of tattooing can be learned but it takes some real practice and some technical skill. Mastering tattoo shading and becoming known for doing great black and grey tattoos can really boost a tattoo artists reputation in the tattoo industry. Those who have mastered tattoo shading at an expert level are very well revered and respected.