Short answer
The binder is the film-forming material that holds pigment particles — linseed oil, egg yolk, gum arabic, acrylic polymer — and it changes colour in two ways. Optically, its refractive index decides how much each particle scatters, so the same pigment looks deep and transparent in oil but pale and chalky in a lean, porous paint. Chemically, it cures, yellows or stays clear over years, and that ageing is often what changes a painting's colour.
Pigment powder has no adhesion and no film; the binder supplies both. In traditional painting the choice defined the medium: drying oils such as linseed, walnut or poppy for oil paint; egg yolk for egg tempera; gum arabic for watercolour and gouache; animal glue for distemper and gesso; and, since the mid-twentieth century, acrylic and other polymer emulsions. Industrial paints use alkyds, acrylics, epoxies, polyurethanes and others. The vehicle is the binder together with whatever liquid thins it — solvent or water — which leaves as the film forms. How that film forms matters: oils cure by reacting with oxygen, emulsions by particles coalescing as water evaporates, gums and glues simply by drying.
Light scatters wherever it crosses a boundary between materials of different refractive index. Most organic binders have an index around 1.5 (acrylic PMMA is about 1.49). A pigment with an index close to that — chalk, most lakes, many earths in oil — scatters little once fully wetted, so the paint is transparent and its colour saturated. The same particles surrounded by air, in a lean or underbound paint, scatter strongly and look paler and chalkier. That is why a dry pigment powder looks lighter than the paint made from it, why a chalk ground is white in glue but goes translucent when soaked with oil, and why damp stone, wet paint and freshly oiled wood look darker than when dry.
Drying oils harden by autoxidation: oxygen attacks the fatty-acid chains next to their double bonds, forming radicals that cross-link the oil into a solid network. The same oxidation continues slowly for decades and produces coloured by-products, so oil films tend to yellow, most visibly in whites and pale blues; conservation guidance notes that this kind of thermal ageing proceeds even in the dark. Egg tempera dries fast into a comparatively matt film, and it does not carry the thick oil-rich layers whose yellowing warms so many later paintings. Acrylic emulsions look milky while wet because the polymer particles scatter light, then clarify as they coalesce — the reason acrylic colours dry darker than they appear on the palette.
The proportion of binder to pigment sets the surface. A binder-rich film levels into a smooth, glossy surface with a single mirror reflection, so colours look deep away from the highlight. A pigment-rich, binder-lean film leaves particles protruding, scattering surface reflection in all directions and adding a whitish veil to every viewing angle — a matt paint looks lighter and less saturated than a gloss paint of identical pigment. This is also why varnishing a matt painting darkens and saturates it, and why a satin sample card may not predict how a matt wall of the same colour reads. The finish pages on this site show the effect with the Colourwise finish simulator.
| Binder | Medium | How the film forms | Colour behaviour |
|---|---|---|---|
| Linseed and other drying oils | Oil paint, some varnishes | Autoxidation and cross-linking | Saturates colour; yellows slowly, more in the dark |
| Egg yolk | Egg tempera | Water loss, then protein and lipid hardening | Fast-drying, relatively matt |
| Gum arabic | Watercolour, gouache | Drying | Very thin; colour relies on paper white |
| Acrylic polymer emulsion | Artists' acrylics, latex house paint | Coalescence as water evaporates | Milky wet, clear dry: colour darkens on drying |
Why: Acrylic and latex emulsions scatter light until the polymer particles coalesce into a clear film.
Fix: Judge colour only on a fully dry sample, ideally a day later.
Why: Oil films keep oxidising slowly, and the products are yellow; the process continues even without light.
Fix: It cannot be painted out by the owner; a conservator can judge whether varnish, dirt or the paint itself is responsible.
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
Drying oils such as linseed harden by autoxidation — oxygen reacting next to double bonds in unsaturated fatty acids, followed by cross-linking — rather than by evaporation.
Source: Drying oil
FactModerate evidence
Acrylic paint is pigment suspended in an acrylic polymer emulsion; most acrylic paints are water-based but water-resistant once dry.
Source: Acrylic paint
Colourwise analysisModerate evidence
At normal incidence, the interface between calcite (extraordinary index 1.486) and a binder of index 1.49 reflects almost nothing, while the same calcite surface in air reflects about 4%; this is why chalk turns translucent in oil but looks white as a dry powder or in a lean paint.
Based on: Fresnel normal-incidence reflectance computed by Colourwise from the cited refractive indices; see the titanium dioxide page's table.
Caveat: Calcite is birefringent and real particles are irregular; the calculation shows the trend, not a measured value.
Source: Calcite; List of refractive indices
FactModerate evidence
Egg tempera is a fast-drying medium of pigment in a water-soluble binder, usually egg yolk, and was the principal European panel-painting medium before oil displaced it.
Source: Tempera
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.