Any intensely coloured compound used to colour other materials. Unlike dyes, pigments do not dissolve; they are applied as fine solid particles mixed with a liquid. In general, the same ones are used in oil- and water-based paints, printing inks, and plastics. They may be inorganic compounds (usually brighter and longer-lasting) or organic compounds. Natural organic pigments have been used for centuries, but today most are synthetic or inorganic. The primary white pigment is titanium dioxide. Carbon black is the most usual black pigment. Iron oxides give browns, ranging from yellowish through orange to dark brown. Chromium compounds yield chrome yellows, oranges, and greens; cadmium compounds brilliant yellows, oranges, and reds. The most common blues, Prussian blue and ultramarine, are also inorganic. Organic pigments, usually synthesized from aromatic hydrocarbons, include the nitrogen-containing azo pigments (red, orange, and yellow; see azo dyes) and the copper phthalocyanines (brilliant, strong blues and greens). Chlorophyll, carotene, rhodopsin, and melanin are pigments produced by plants and animals for specialized purposes.