Short answer
Usually because it has not finished drying, because one coat is not hiding what is underneath, or because patches of the wall dried with a different sheen — not because the wrong colour was mixed. Which of these it is depends on when you noticed, whether the difference is patchy or even, and whether it changes with the angle you look from.
1. The paint has not finished drying and curing
Test: Look at the wall at the same time of day two or three days later, and compare an area painted first with one painted last. If the difference is shrinking, it is drying.
A wet film is darker and glossier than the dry one, and emulsions keep changing slightly for days as the last water leaves and the surface texture forms. Porous plaster dries in patches, so a wall can be several colours at once for a while.
2. Not enough coats: the old colour or the filler is showing through
Test: Hold a sheet of white paper against the wall in several places, then look at patched and unpatched areas at a low angle. Shadows of the old colour, or lighter squares over filler, mean the film has not hidden the base.
Pale colours and clean yellows, reds and oranges hide poorly; one coat over a darker or patchy base is a mix of the new paint and what is underneath.
3. Uneven sheen ('flashing') from a porous or patched base
Test: Stand beside the wall and look along it towards the window. Patches that look shinier or duller than their surroundings, often matching filled holes or cut-in edges, are sheen differences, not colour differences.
Unsealed filler and bare plaster absorb binder, leaving a flatter patch; roller and brush areas dry with slightly different texture. A duller patch looks lighter and chalkier; a glossier one looks deeper.
4. You are comparing it with a card or sample seen under different light
Test: Take the colour card or sample board to the wall and hold it flat against the paint. If card and wall match when touching but not in memory, the paint is right and the expectation was set elsewhere.
A small card near a window is judged under more light and against a white border; a wall is judged at room scale with inter-reflection. The finished room almost always reads deeper.
5. The tinted batch differs from the one you sampled
Test: Paint a stripe from each tin side by side on white card, let it dry fully, and compare. Check the batch or tint labels on the tins.
Tinting machines and base batches vary slightly; two tins of the same named colour can differ enough to show on a continuous wall, especially in deep colours.
Causes are ordered by how often Colourwise expects each to be the explanation — an editorial judgement, so test them all.
Timing separates the causes faster than anything else. A difference noticed within hours of painting is almost always drying: the wet film is darker and glossier, and on new or porous plaster the wall dries in blotches that can last days. A difference that is still there after the drying time on the tin, and looks like shadows, stripes or squares, is coverage — the base is showing through. A difference that is only visible from certain angles is sheen. A difference that only appears in the evening is the light, and belongs on a different page. And a wall that is evenly, stubbornly 'not the card' after a week is either a batch problem or, far more often, an expectation set by a small card viewed somewhere else.
New gypsum plaster needs weeks to dry out before it takes paint evenly; lime plaster far longer. Painting too early produces blotches that look like colour faults and are really moisture.
Two faults produce patches and they need different fixes. Show-through is a film too thin to hide the base: pale colours and clean yellows, oranges and reds hide poorly, so a single coat over a darker old colour or grey filler reads as a mix of both. It looks the same from every angle. Flashing is a sheen fault: filler and bare plaster soak up binder and dry flatter than the painted areas round them, and cut-in edges dry with a different texture from rolled areas. Flashed patches change with angle — duller and paler from one side, similar from the other. A torch held flat against the wall shows flashing clearly; a sheet of white paper held against the wall shows show-through. Only show-through is cured by simply adding paint; flashing needs the patches sealed or a full, even coat corner to corner.
If every part of the wall looks the same but not what you expected, test the paint before blaming it. Brush a stripe from each tin onto white card, let it dry completely, and hold it against the wall. If card and wall match, the paint is right and the difference is scale and setting: a finished room reads deeper than a chip because the walls reflect colour onto each other, which the practical guide to paint looking darker on the wall covers in detail. If stripes from two tins differ, the batches or tints differ, and mixing all the tins together before painting is the fix. If your dried stripe differs from the maker's printed card, remember that the card is printed ink, not paint — compare with a painted sample or the retailer's reference instead.
Most repainting after a colour 'surprise' is done too early. Adding a coat because the wet paint looks too dark wastes a coat and can trap moisture in fresh plaster. Touching up small patches with a brush on a rolled matt wall usually creates new flashing, because the brushed paint dries with a different texture and sheen even from the same tin — a full wall, corner to corner, is the unit of repair for matt finishes. Starting a new tin halfway across a wall invites a visible line if the tins are from different batches. And judging a sample on a patch of the old wall colour, rather than a board, distorts it through the surrounding contrast before you have even started.
Why: Brushed touch-ups dry with a different texture and sheen from the rolled surface around them
Fix: Repaint the whole wall section from corner to corner with a roller, or keep touch-ups tiny and feathered
Why: Unsealed filler absorbed binder and dried flat, or the grey filler is showing through
Fix: Spot-prime the filled areas, then apply a full coat over the whole wall
Why: A new tin from a different batch was started mid-wall
Fix: Mix all tins together before starting, or change tins only at a corner
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 surface water film traps diffusely scattered light by total internal reflection, increasing absorption.
Caveat: The study models rough absorbing surfaces generally; how much a particular emulsion changes on drying also depends on its binder and sheen.
Source: Why some things are darker when wet (Applied Optics 27, 1278)
FactStrong evidence
Gypsum plaster sets in minutes but typically takes two to three weeks to dry, while historic lime plaster could take more than a year.
Source: Preservation Brief 21: Repairing Historic Flat Plaster — Walls and Ceilings
FactModerate evidence
In Kubelka–Munk terms a pale, weakly absorbing coating needs a much greater thickness than a deep colour to hide a contrasting background, which is why pale colours show the old colour through a single coat.
Source: New Contributions to the Optics of Intensely Light-Scattering Materials. Part I (Journal of the Optical Society of America, 1948); Kubelka–Munk theory
Colourwise interpretationLimited evidence
Most complaints that a newly painted wall is the wrong colour are resolved by full drying and a second full coat, rather than by a different paint.
Based on: Colourwise reading of the drying and hiding mechanisms above together with common decorating guidance to judge colour only when dry and to apply two coats.
Caveat: Not a measured frequency. Tinting errors do occur, and persistent dark patches can be damp rather than paint.
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.