Place dry pigment on the work surface and create a well in the middle to act as a reservoir. Add a small amount of water to the reservoir and slowly mix the water and pigment using a painter's spatula.
Continue to add water to the pigment while mixing to create a thick paste. Ideally, the amount of paste should equal the amount of acrylic paint used as a base. When using organic pigments, add a small amount of methyl or grain alcohol to the paste to aid in dispersion. Organic pigments have an electrical quality that repels water.
Grind the paste gently with a glass muller while adding small amounts of water until all particles are well mixed and the paste has a creamy texture. In place of a muller, you can use a small drinking glass with a thick heavy base. Wear gloves and hold the glass close to the base while grinding to prevent breakage.
Add the paste to acrylic paint and mix thoroughly until paint has an even tone. Avoid mixing organic and inorganic pigments to prevent muddy colors. Use organics if you're unsure about the pigment content in the acrylic paint.