Colourant

A colorant or color additive is a substance that is added or applied in order to change the colour of a material or surface. Colourants can be used for many purposes including printing, painting, and for colouring many types of materials such as foods and plastics. Colourants work by absorbing varying amounts of light at different wavelengths (or frequencies) of its spectrum, transmitting (if translucent) or reflecting the remaining light in straight lines or scattered.

Most colourants can be classified as dyes or pigments, or containing some combination of these. Typical dyes are largely translucent, while pigments are made up of solid particles and are partially or entirely opaque. These properties may change when other common ingredients such as binders and fillers are added, for example in paints and inks. In addition, some colourants impart colour through reactions with other substances.

Colourants, or their constituent compounds, may be classified chemically as inorganic (often from a mineral source) and organic (often from a biological source).

Origin of the coloring of a substance
The coloring of a substance is due to absorption or reflection by it of light radiation of specific wavelengths. At the molecular level, the absorption of light energy in the form of the quantum of light, the photon, results in electronic transitions. The excited electron goes to an energy level higher than that of the ground state, and the electronic transition is followed by the emission of transmitted light, which appears colored by subtraction of the radiation of certain wavelengths. In general, the absorbed energy is stored in the form of heat, diffuses gradually into neighboring materials and dissipates into infrared radiation. In organic dyes, a certain structural configuration of the molecule is necessary to obtain color.

Groups of atoms, more or less complex, with many double bonds, are responsible for the coloring: they are chromophores. The most important are the nitro group (-NO2), linked to an aromatic ring, the azo group (RN = N-R ‘), linked to two aromatic rings, the stilbene group (C6H5-CH = CH-C6H5), the triarylmethane, acridine, quinoline, anthraquinone, indigo, etc.

Pigments and dyes
The definitions of the words dye, pigment or dye differ according to the fields of application.

In the field of dyeing, pigments and dyes are particular dyes:

the pigments are insoluble and without affinity for the support, they can color the support on the surface if a binder (resin, etc.) is added;
dyes are soluble dyes absorbed by the support, which often require a fixative.
In biology, a pigment is an organic molecule that communicates its color to various tissues and body fluids (chlorophyll, hemoglobin, etc.).

In chemistry :

a pigment is an insoluble substance that can stain a surface when it is mixed with a binder;
a dye is a soluble substance that gives color to the solution.

Regulation
In the US, the Food and Drug Administration (FDA) regulates colourants for food safety and accurate labeling.

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