Short answer
Bare plaster takes its colour from its binder — white lime or gypsum — and from the sand, marble dust or pigment mixed with it, and it looks much darker while wet than when dry. Gypsum dries within a few weeks, lime can take many months, so colour should only be judged once drying is complete. After that, plaster's colour moves with soiling, damp and salts rather than fading, and any finish applied over it — limewash, mineral paint, emulsion — sits on a porous, highly scattering base that affects how that finish looks.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Drying and carbonation | Patchy dark areas fading to an even pale finish | Water leaving the pores; lime also hardens by taking up carbon dioxide | Judge colour and decorate only after full drying |
| Soiling and salt movement | Grey soiling, white salt bloom, tide marks from damp | Porous surface drawing in dirt and dissolved salts | Cure the damp source; use breathable limewash or mineral paint |
Traditional plaster used lime putty with sand and animal hair for the base coats, and a finish coat — 'setting stuff' — that was mostly lime with little aggregate, giving a smooth white face; marble dust could be added for a hard, polishable white finish, and fine white sand for a float finish. From around 1900 gypsum plaster took over because it sets in minutes. Both binders are white; the tone of the finished wall comes from what else is in the top coat. External stucco historically took its colour mostly from its sand, sometimes from natural earth pigments or brick dust, so it tended to warm earth tones until factory-pigmented stuccos appeared in the 1920s. Modern tinted and polished plasters (marmorino, Venetian) carry pigment through the thickness, so scratches and chips expose the same colour rather than white.
Wet plaster is dark because water fills its pores and suppresses internal scattering. As it dries, the surface lightens from the edges and from thin areas first, leaving darker islands over thick patches, cold spots and backgrounds that hold moisture. Gypsum plaster dries in two to three weeks in reasonable conditions; historic lime plaster could take more than a year, which delayed decoration. A newly plastered wall that looks patchy, pinkish-brown or blotched is usually telling you about moisture, not pigment. Painting too early traps the water: the emulsion can blister or flash, and a tinted finish can dry unevenly because the plaster below is still changing.
Bare dry plaster is highly porous and absorbent. A first coat of emulsion soaks in unevenly and dries matt in some areas and with more sheen in others — the 'flashing' that a mist coat is meant to prevent. Limewash and mineral silicate paints bond into the surface and stay translucent enough to let its variation show; that soft, cloudy depth is the reason people choose them. Polished plasters are burnished so the surface is smooth enough to show a satin reflection over a pigmented body, which reads darker and richer than the same pigment in a matt finish. The finish-guide pages on limewash and Venetian plaster cover application; the point here is that the plaster is the substrate those finishes rely on.
The lime and gypsum in plaster are mineral and do not fade in light. What changes plaster colour is water. Damp walls draw dissolved salts to the surface, where they crystallise as white bloom or leave tide marks at the height the damp reaches. Condensation and damp encourage mould, which greys or blackens corners. Porous surfaces also hold soot and dust, so plaster near radiators and above lamps darkens in streaks that follow air movement. Gypsum plaster is more vulnerable to wetting than lime; where plaster keeps getting wet, discolouration returns after cleaning or painting until the source of the moisture is dealt with.
Why: Uneven absorption of a porous surface, or plaster not yet dry.
Fix: Let it dry fully, then apply a thinned mist coat before the finish coats.
Why: Different absorbency and texture from the surrounding wall.
Fix: Seal the patch, feather its edges and paint the whole wall section.
Why: Salts carried out by damp and crystallising at the surface.
Fix: Brush off dry; find and cure the moisture source before redecorating.
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
Historic three-coat lime plaster used a finish coat with a high proportion of lime putty, little aggregate and no fibre, which gave a smooth white surface; marble dust could be added for a hard white finish.
Source: Preservation Brief 21: Repairing Historic Flat Plaster — Walls and Ceilings
FactStrong evidence
Gypsum plaster sets within minutes and dries in two to three weeks, whereas a lime-plastered wall could take more than a year to dry.
Source: Preservation Brief 21: Repairing Historic Flat Plaster — Walls and Ceilings
FactStrong evidence
The colour of most early stucco came from the sand in the mix, with natural pigments, limewash or colourwash sometimes added, producing mainly earth tones until factory-pigmented stuccos in the twentieth century.
Source: Preservation Brief 22: The Preservation and Repair of Historic Stucco
FactStrong evidence
Colour matches for stucco repairs should be judged on dried test samples, ideally left to weather for a year or at least through a change of seasons beside the original.
Source: Preservation Brief 22: The Preservation and Repair of Historic Stucco
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.