Pigment dispersions for medical tattooing
The pigment is the ingredient (or raw material) that gives ink its colour. While it is an essential component, it is not sufficient on its own. As pigments come in powder form, they cannot be easily placed in the skin. It is necessary to make a liquid colorant, which is easy to implant.
By definition, pigments are not soluble in the usual liquids (this property is the reason we use them, as otherwise they would dissolve in the skin), so they are placed in a "dispersion" (suspension) in a liquid called a "dispersant". Remember that colorants (for food) are soluble, so they are placed in a homogeneous "solution" with a "solvent".