Short answer
A dye dissolves in the material it colours, or in the liquid that carries it, and ends up as single molecules bound to or dispersed through that material; a pigment stays as solid particles that do not dissolve. The difference is about behaviour in a particular medium rather than chemistry alone: the same coloured compound can act as a dye in one system and, once made insoluble, as a pigment in another.
The working definition used by the colourant industry is about solubility in the application medium. A dye is soluble in, or goes into solution in, the liquid used to apply it — water in a dyebath, a solvent in an ink, the polymer melt in some plastics — and ends up molecularly dispersed, often chemically or physically attached to the fibre or substrate. A pigment is insoluble in its vehicle and is dispersed as particles, usually somewhere between tens of nanometres and a few micrometres across, held in place by a binder. That single distinction explains most of the practical differences between the two: how they are applied, how transparent they are, how they migrate, and how they age.
A dissolved dye can only absorb: each molecule removes certain wavelengths and lets the rest through, so a dyed film or fibre is essentially a coloured filter over whatever lies beneath. Pigment particles absorb too, but because they are separate solids with their own refractive index they also scatter light at every particle surface. That is why a pigment can hide what is underneath while a dye cannot, and why the same colourant chemistry looks cleaner and more transparent as a dye and more opaque as a coarse pigment. Scattering depends on particle size and on the difference in refractive index between particle and binder — the subject of the titanium dioxide and opacity pages in this section.
Dyes and pigments are catalogued in the Colour Index, published by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. Each essential colourant gets a Generic Name such as Pigment Blue 15 or Acid Red 18 and, where the structure is disclosed, a Constitution Number. Part 1 covers pigments and solvent dyes; Part 2 covers dye application classes — acid, basic, direct, disperse, reactive, vat, sulphur and others, including fluorescent brighteners. The publishers warn that products sharing one Generic Name may still differ in fastness and application behaviour, which is why an artist's or ink maker's label naming the pigment is useful but not a guarantee of identical performance.
Several important colourants sit on the line. A lake pigment is a dye made insoluble by precipitating it, usually with a metal salt and often onto a colourless substrate, turning a dye into a pigment. Indigo is applied to cloth in a soluble, colourless reduced form and then oxidises back to an insoluble coloured solid inside the fibre, so it enters as a dye and ends as something closer to a pigment. Solvent dyes are soluble in plastics and oils but not water. And a pigment ground fine enough scatters so little that it behaves optically like a dye, which is exactly what transparent pigments for inks and car basecoats exploit.
Because a dye is spread out as individual molecules, every molecule is exposed to light, oxygen and pollutants, and once a molecule breaks the colour it carried is gone. A pigment particle has most of its molecules buried inside the crystal, shielded by the outer layers, so the same chemistry is usually more lightfast as a pigment than as a dye. The trade-off is in brightness, transparency and ease of application. The pattern shows up across this site: dye-based inkjet prints fade faster than pigment prints, lake pigments made from fugitive dyes fade in old paintings, and textile dyes are among the most light-sensitive colourants a museum looks after.
| Property | Dye | Pigment |
|---|---|---|
| State in the medium | Dissolved; molecular | Solid particles in suspension |
| What holds it | Affinity or bonding to the fibre or substrate | A binder that surrounds the particles |
| Scattering | None of its own | Yes, depending on particle size and refractive index |
| Hiding a background | No — acts as a filter | Possible, if particles scatter enough |
| Migration and bleeding | Can move with water, heat or solvents | Much less prone, unless partly soluble |
| Typical uses | Textiles, paper, dye inks, transparent plastics | Paints, most printing inks, coloured plastics, cosmetics |
Why: Many markers and felt-tips use dyes, which sit in the high light-sensitivity category.
Fix: Use pigment-based pens for anything to be displayed, and keep the work out of direct daylight.
Why: A Colour Index name identifies the colourant, not the particle size, surface treatment or grade.
Fix: Treat the pigment name as a starting point and test the specific product for strength and lightfastness.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
A pigment is insoluble in its vehicle and forms a suspension, while a dye is itself a liquid or dissolves in its vehicle; the same colourant can act as a pigment or a dye depending on the vehicle.
Source: Pigment
FactStrong evidence
The Colour Index assigns Generic Names and Constitution Numbers to dyes and pigments, in two parts: pigments and solvent dyes, and dye application classes; products under the same Generic Name may not have identical fastness properties.
FactModerate evidence
Pigment-based inkjet inks provide much better long-term durability than dye-based inks, most of which fade quickly in light or ozone.
Caveat: Ink formulations vary widely; some modern dye inks perform much better than early ones.
Source: Inkjet printing; Bleed — DP3 Digital Print Preservation Portal
Colourwise interpretationModerate evidence
Pigments are generally more lightfast than dyes of similar chemistry because most colourant molecules in a pigment particle are shielded inside the crystal, whereas every molecule of a dissolved dye is exposed.
Based on: Colourwise reading of the pigment/dye distinction (solid particle versus molecular dispersion) together with the consistently faster fading of dye-based inks and textile dyes in the cited IPI and CCI material.
Caveat: Many exceptions: some dyes are very lightfast and some pigments are fugitive.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.