A pigment must have properties of high resistance to dyeing and be stable in solid form at room temperature. The selection of the pigment for a particular application depends on its cost and the requirements of the physical properties and attributes of the pigment.

Inorganic pigment and organic pigment

The pigments are insoluble and are applied not as solutions but as finely ground solid particles mixed with a liquid. In general, the same pigments are used in oil and water based paints, printing inks and plastics. The pigments can be organic or inorganic. Most inorganic pigments are brighter and last longer than organic pigments. Organic pigments made from natural sources have been used for centuries, but most of the pigments used today are inorganic or synthetic. Synthetic organic pigments are derived from coal tars and other petrochemical products. Inorganic pigments are produced by relatively simple chemical reactions, particularly oxidation, or are found naturally as earths.

Inorganic pigments
Inorganic pigments include white opaque pigments used to provide opacity and to lighten other colors. The white extension pigments are added to the paints to reduce their cost or improve their properties. This class includes calcium carbonate, calcium sulfate, diatomaceous silica (marine organisms) and porcelain clays. Black pigments are created mainly from carbon particles. Carbon black, for example, is used to black print inks. Iron oxide earth pigments produce ocher (yellow-brown), siennas (orange-brown) and umbrella (brown). Certain chromium compounds are used to provide yellows, oranges and chrome greens, while several cadmium compounds produce yellows, oranges and bright reds. Iron, or Prussian blue, and ultramarine blue are the most commonly used blue pigments and are of inorganic origin.

Organic pigments
For the most part, organic pigments are currently synthesized from aromatic hydrocarbons. These are compounds that contain structures of carbon atoms with attached hydrogen atoms that form in closed rings. The organic pigments include azo pigments, which contain a nitrogen group; they represent most of the red, orange and yellow organic pigments. Copper phthalocyanines offer bright, strong blues and greens that are unusually resistant to organic colors. Some pigments, such as fluorescents, are simply dyes that have become insoluble by chemical reaction.