Before you can create any realistic shading technique, you need to understand the basics of artistic shading. This is the same for tattoos and other works of art. Start by choosing the direction of your light source. Once you know the location of your light source, you can place the shadows on the tattoo appropriately. For example, if the tattoo's light source comes from the upper left corner of the picture, your tattoo's shadows will be on the lower right side.
When color ink is not available or desired, a tattoo artist can dilute the pigment of black ink to create a shadowed effect on a tattoo. The tattoo artist uses gray colors for the light areas of the tattoo. He will progressively darken the tattoo until it reaches a shadowed gray color or a complete black. This style requires patience and care or the shadows will looks blotchy and uneven. Most tattoo artists aim for a smooth transition from light to dark when creating a tattoo.
More ink creates a bolder, darker color. To get more ink into the tattoo site, the tattoo artist must bore deeper into the skin. Lighter areas don't require such depth. To create shading on a tattoo, an artist needs to adjust the puncture depth on the tattoo gun. A tattoo artist can make the shading in this way without the need to dilute the ink. Adjusting the depth of the needle is something only an experienced tattoo artist should attempt, though. If the depth isn't just right, it can ruin the tattoo.
If a realistic look and smooth lines aren't important to a client, you can create shading by making small dots on the artwork. As the shadow intensifies, the dots in the tattoo will increase in size. Black ink works best for this shading since it is more noticeable than a light colored ink. Your ink color can go from a light gray to a darker ink as well if you don't want all of your tattoo's shading to be black.