Pigments and dyes are called colorants. The ways in which colorants are used determines whether they are pigments or dyes. Pigments are water-and oil-insoluble natural and synthetic products that impart color to materials such as paper and plastics. Dyes, by contrast, are water-soluble colorants, although some are converted into insoluble lake pigments by coprecipitating onto an inorganic base. Artists' colors are pigments that are spread on a surface suspended in a suitable medium, such as oil. The mass coloration of textile fibers, polymers, plastics, and rubber takes place when pigments exist in the form of dispersions. A convenient way of classifying pigments is into organic and inorganic pigments.