Tattoos are forever, and for that reason it's important that they be done as skilfully as possible. While there are a number of ink colours available to use, sometimes you will have to mix colours or thin a colour to create just the right shade to use for certain stylistic touches, such as shading, which can give a flat-looking tattoo depth and a very professional finish.
Set out a number of unused inkwell cups. Fill each cup with a small amount of ink. The more ink you put in the cup, the more difficult it will be to thin it out.
Mix a small amount of distilled water to the first of the cups. The ink will be thinner and lighter. This technique is often used to give a wash look to tattoo shading.
Mix different amounts of distilled water into the inkwell cups so see how thin and light it makes the colour. Stir the ink and water well to make sure that they are thoroughly mixed.
Continue mixing -- in different cups, if possible, to keep track of the water-to-ink ratio of each attempt -- until the ink has reached your desired shade.
Create as much of your desired ink mixture as you will need for the specific tattoo.
Since there is no exact formula, and tattoos are all about personal preference, it's essential to continue trying different amounts of thinning in order to find one that provides your desired effect. Mixing a little bit of white ink into the ink will lighten it without diluting it.
For health reasons, use only sterile materials and tools when thinning tattoo ink.