A soluble colouring material that chemically bonds with or is absorbed into the material it colours, such as fabric.
Unlike a pigment, a dye dissolves into the substrate rather than remaining as discrete suspended particles — this generally makes dyes suited to fabric and pigments suited to paint, though some materials use both.
'Dye' and 'pigment' are often used interchangeably in everyday speech, but they are physically different (see pigment).
Indigo and cochineal are historically significant dyes — see the pigments & dyes section for structured records.
Understanding pigment vs dye explains why some historical colourants suited textiles and others suited paint or ink.
Dyes are classed by how they attach to a fibre, with reactive, direct and vat dyes the main groups for cotton, and are listed by Colour Index generic name. How well the result survives is rated by fastness tests, each reported separately: fastness to light, to washing and to rubbing. A dye can be excellent on one and poor on another, as indigo on denim is, and a reactive dye that bonds chemically to the fibre outlasts a direct dye that is held only by weak physical forces.
A dark cotton garment is washed with pale ones for the first time.
Dye that never fixed to the fibre washes out and is picked up by the paler fabric. The washfastness rating predicts it, and the garment's first washes should be alone and cool.
Curtains or upholstery are placed in a sunny window.
Every molecule of a dissolved dye is exposed to the light, so dyes generally fade sooner than pigments of similar chemistry. The side facing the glass goes first and the fold lines stay dark.