Color is one of the many ways to enhance a tattoo--using it incorrectly is worse than not using it at all. Mixing ink colors is an all-or-nothing deal and if done incorrectly will leave a horrible mess on your skin that you will have to live with unless you have the money for tattoo removal. Tattoo ink exists in two forms: water-based and pure pigment. Pure pigment inks are alcohol-based and cannot be mixed--they are considered true color and mixing them would mean mixing different chemical properties that could become toxic. Water-based inks can be mixed.
Cut a strip of cellophane or plastic wrap and lay it on your work space. This is the only thing that you want to mix on as nothing will be transferred from the surface.
Place a small amount of ink 1 on the cellophane. Don't use a large amount as you don't want to waste it because it is expensive. Make sure there is enough to do all of the section you are tattooing in that color because it's difficult to make the exact same shade more than once unless you're highly experienced.
Place the same amount of ink 2 on the cellophane next to ink 1. Use more or less if you are trying to make a darker or lighter shade.
Mix the two colors together with your toothpick. Don't touch it with your skin. Make sure that you thoroughly mix all of the color together so that it is the correct shade you want.