Casein colors are paints made by mixing artists' pigments with a solution of Casein, a milk protein. They are usually applied with a brush on a wall-board panel coated with gesso (Claybord or Gessobord). Canvas cannot be used, because dry casein is the most inflexible of the permanent paint binders and becomes increasingly brittle as it ages. For the same reason, the palette knife should be used with restraint, as the paint will crack if laid on too thickly; a heavy impasto should be avoided. The colors dry rapidly to a pleasing and durable mat finish, although some artists varnish their pictures to obtain a glossy finish more characteristic of oil paints. Caseins may be used to produce effects ranging from smooth areas of flat color to the robust textures of semi-impasto, for which a full bristle brush is used. Casein is also frequently used as an underpainting for oil paints and glazes.

Besides the mat quality of casein colors and their high drying speed, some artists prefer them to oils because they can be mixed with water. Although dried casein paint may be damaged if spattered with water, it is not readily water-soluble and will resist dampness. A casein painting may be sprayed with Formalin (or spray fixative like Krylon® UV Resistant Clear Coating #1309 (Matte) or #1305 (Gloss) to increase its resistance to moisture. It may be cleaned with Acetone. In the later 1940's, manufacturers of artists' materials brought out complete lines of improved casein colors in tubes and the technique became quite popular in the United States.