Short answer
A dry porous or fibrous material is full of air gaps, and every boundary between solid and air scatters a little white light back out, so the material looks paler than its substance really is. Water fills those gaps; because water's refractive index is close to that of most minerals and fibres, the boundaries almost stop reflecting, light travels deeper and is absorbed more, and a water film on top traps scattered light by total internal reflection. The colour darkens and saturates — the more porous the material and the smaller the refractive-index step, the stronger the effect.
The colour of a sand grain, a cotton fibre or a crystal of calcite is set by what it absorbs. What makes a heap of grains or a bundle of fibres look pale is scattering: at every surface between solid and air a few per cent of light reflects, uncoloured, and a porous material has an enormous number of such surfaces near its face. Much of the returning light has therefore only visited the outer grains and picked up little colour. This is also why powdered or crushed materials look lighter than the solid — ground coloured glass is paler than the lump — and why a scratch on a dark plastic looks white.
Reflection at a boundary depends on the difference in refractive index on either side. Between air (1.0) and a silica-like mineral (about 1.46) roughly 3.5% of light is reflected at normal incidence; with water (1.33) in place of air, the figure falls to about 0.2%. The table on this page shows the same calculation for other materials: for most minerals and fibres the internal reflections fall by a factor of roughly seven to seventeen. With less light turned back near the surface, more light penetrates, passes through more coloured material and is absorbed. Lekner and Dorf's 1988 analysis added a second effect: a water film on top lets scattered light be totally internally reflected back into the surface, giving it another chance to be absorbed.
The effect scales with porosity and the size of the index step. Sand, soil, limestone, sandstone, unglazed terracotta, concrete, bare plaster, cotton, linen and paper darken strongly. Dense, low-porosity materials — polished granite, glazed tiles, sealed wood, glass — hardly change, because there are few internal gaps for water to fill. White paint changes little because its scattering comes from titanium dioxide particles already embedded in binder, not from air gaps; and titanium dioxide's index is so high that even water leaves a large step, as the rutile row of the table shows. Clear finishes behave like permanent water: an oiled, varnished or 'wet-look' sealed surface shows the darker, richer colour the material had when wet.
Materials do not dry evenly. New plaster and render show dark patches where they are thicker or on colder backgrounds; concrete and paving dry first at edges and over voids; bricks dry in patterns that trace mortar and cavity. These patches are informative — they show moisture, not colour defects — and the colour should be judged only when everything is dry. The corollary for specification is that any sealer or impregnator that fills pores will shift a porous material towards its wet colour, sometimes only slightly, sometimes strongly; 'colour enhancing' sealers do this deliberately. Always test a sealer on an offcut and let it cure before judging.
| Particle | Reflected in air | Reflected in water | Reduction |
|---|---|---|---|
| Silica-like mineral (n ≈ 1.46) | 3.47% | 0.20% | 17.3× |
| Glass or cellulose-like solid (n ≈ 1.52) | 4.26% | 0.43% | 9.9× |
| Polyester-like fibre (n ≈ 1.58) | 4.99% | 0.69% | 7.2× |
| Rutile pigment particle (n ≈ 2.61) | 19.94% | 10.53% | 1.9× |
Why: It is still drying; thicker and colder areas hold water longer.
Fix: Wait until fully dry before judging colour or painting.
Why: The sealer filled pores, shifting it towards its wet colour.
Fix: Test sealers on an offcut; choose a non-darkening impregnator if the dry colour is wanted.
Why: It was matched while damp, when it looked darker and more saturated.
Fix: Always compare fully dry and conditioned fabric.
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.
FactStrong evidence
Rough absorbing materials look darker when wet partly because a water film allows diffusely scattered light to be totally internally reflected back into the material, increasing absorption, and partly because the smaller refractive-index step between water and the material reduces scattering.
Source: Why some things are darker when wet (Applied Optics 27, 1278)
Colourwise analysisModerate evidence
Replacing air with water around a silica-like particle cuts normal-incidence reflection at its surface from about 3.5% to about 0.2%, while for a rutile titanium-dioxide particle the reduction is only about twofold.
Based on: Fresnel normal-incidence reflectance computed by Colourwise from listed refractive indices of water, fused silica and rutile; see the table on this page.
Caveat: Normal incidence only; real particles present all angles, which changes the numbers but not the direction.
Source: List of refractive indices; HyperPhysics (optics, atmospheric optics and vision pages)
FactStrong evidence
Historic lime plaster could take more than a year to dry, while gypsum plaster sets in minutes and dries in two to three weeks.
Source: Preservation Brief 21: Repairing Historic Flat Plaster — Walls and Ceilings
FactStrong evidence
Porosity of sedimentary stones such as limestone and sandstone can be as high as about 20%, while granite's pore volume is very low.
Source: Preservation Brief 48: Preserving Grave Markers in Historic Cemeteries
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.