Short answer
A paint hides a surface when light entering the film is scattered back out or absorbed before it reaches what is underneath and returns. Kubelka–Munk theory describes this with two coefficients, absorption (K) and scattering (S): hiding needs enough of either multiplied by film thickness. Whites and pale tints rely on scattering alone and need thick films; deep colours absorb strongly and hide in thin films; clean yellows and reds made from transparent organic pigments do neither well.
In 1931 Paul Kubelka and Franz Munk modelled a paint film as two streams of diffuse light, one travelling down and one travelling up, each losing energy to absorption at rate K and exchanging energy with the other by scattering at rate S. The model has two useful results. For a film so thick that the substrate no longer matters, reflectance depends only on the ratio K/S — the basis of colour matching software. For a film of finite thickness X over a background, reflectance depends on K/S, on the product SX and on the background's own reflectance, which is exactly the question of hiding: how thick must a film be before the substrate stops showing? The theory's assumptions — perfectly diffuse light, no surface reflection, particles much smaller than the film — make it approximate, but its structure is why the industry thinks about opacity in these terms.
Hiding is measured by drawing the paint down at a known thickness over a chart with black and white areas and measuring its reflectance (usually the CIE Y value) over each. The contrast ratio is reflectance over black divided by reflectance over white. A film that completely hides scores 100%; a contrast ratio of 98% is a commonly used threshold for complete hiding. The ratio depends on thickness, so a hiding-power figure is always tied to a film thickness or spreading rate — square metres per litre at which a given contrast ratio is reached. That link is why a manufacturer's coverage claim and a decorator's number of coats can disagree: rollers apply thinner films than laboratory draw-downs, and walls are rarely a uniform white.
The Colourwise calculation below uses Kubelka–Munk to show the pattern. A white with K/S of 0.005 — almost no absorption — needs a scattering thickness SX of about 9 to reach a 98% contrast ratio. A pale tint needs a little over half that, a mid-tone under a quarter, and a deep colour about one eighteenth, because absorption finishes the job scattering starts. Painters and decorators see the practical side: a bright white over a dark red needs several coats, a navy covers almost anything in one or two. The exception is colours made from transparent, strongly absorbing organic pigments with little white in them — clean lemon yellows, oranges and some reds — which absorb only part of the spectrum and scatter little; they can need many coats over a contrasting base unless a matched opaque primer goes on first.
Scattering rises with the refractive-index difference between pigment and binder and peaks at particle sizes around half a wavelength, which is why titanium dioxide dominates and why flocculated, clumped pigment hides less than well-dispersed pigment. Absorption rises with pigment concentration and strength. Film thickness multiplies both, so a thin second coat adds more hiding than extra stirring. Matt paints often hide slightly better per coat than glossy ones because air-filled voids and extenders add scattering surfaces. And the substrate matters as much as the paint: a grey or tinted primer halves the contrast a light topcoat has to overcome, which is the reasoning behind tinting a primer towards the finish colour.
| Scattering thickness SX | White (K/S 0.005) | Pale tint (K/S 0.05) | Mid-tone (K/S 0.5) | Deep colour (K/S 5) |
|---|---|---|---|---|
| 0.5 | 40.8% | 41.7% | 50.2% | 97.3% |
| 1 | 60.2% | 62.4% | 79.6% | 100.0% |
| 2 | 78.3% | 82.4% | 97.5% | 100.0% |
| 4 | 90.8% | 95.4% | 100.0% | 100.0% |
| 8 | 97.2% | 99.7% | 100.0% | 100.0% |
| 16 | 99.6% | 100.0% | 100.0% | 100.0% |
| Colour (K/S) | Reflectance of a thick film | SX for 98% contrast ratio | Relative to white |
|---|---|---|---|
| White (K/S 0.005) | 90.5% | 9.30 | 100% |
| Pale tint (K/S 0.05) | 73.0% | 5.26 | 57% |
| Mid-tone (K/S 0.5) | 38.2% | 2.10 | 23% |
| Deep colour (K/S 5) | 8.4% | 0.52 | 6% |
Why: Clean yellows use transparent organic pigments that absorb and scatter too little to hide a darker base.
Fix: Prime with a light grey or a matched opaque undercoat first; then two topcoats usually suffice.
Why: The claim assumes a laboratory film thickness; rollers spread thinner, and porous or dark walls need more.
Fix: Apply at the stated spreading rate (litres per square metre), and budget an extra coat over strong contrasts.
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
Kubelka–Munk theory, published in 1931, addresses how a coat of paint of given composition and thickness changes the colour of a substrate and the thickness needed to obscure it, and gives a definition of hiding power.
Source: Kubelka–Munk theory; New Contributions to the Optics of Intensely Light-Scattering Materials. Part I (Journal of the Optical Society of America, 1948)
Colourwise analysisModerate evidence
In Kubelka–Munk terms, a white of K/S 0.005 needs a scattering thickness about 18 times that of a deep colour of K/S 5 to reach a 98% contrast ratio over a black-and-white background.
Based on: Colourwise solution of the finite-layer Kubelka–Munk equation for contrast ratio over backgrounds of 0% and 80% reflectance; see the tables.
Caveat: Assumes equal scattering coefficients and ideal diffuse conditions; real paints differ in S as well as K.
Source: New Contributions to the Optics of Intensely Light-Scattering Materials. Part I (Journal of the Optical Society of America, 1948); Kubelka–Munk theory
FactModerate evidence
Titanium white provides greater hiding power than any other white pigment, and its hiding power made it economical because less pigment is needed for the same opacity.
Source: Titanium white
Colourwise interpretationModerate evidence
Bright, clean yellows, oranges and reds made from organic pigments are typically the hardest decorative colours to cover with, because their pigments absorb only part of the spectrum and scatter little.
Based on: Colourwise reading of Kubelka–Munk behaviour for low-K/S, low-S films together with the transparency of organic and lake pigments described elsewhere in this pillar.
Caveat: Products formulated with opaque inorganic pigments or extra white hide much better, at the cost of cleanliness of colour.
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.