Three color channels are not enough to discriminate between all natural surface reflectances.

True: colour-science literature shows that distinct natural surface reflectance spectra can produce identical responses in a three-channel (trichromatic) system under the same illuminant (metamerism), so three channels cannot tell all natural reflectances apart. Mostly true: this holds in principle but natural metamers are rare, and the claim is still accurate as stated. Mixed: the literature is split on whether natural reflectances produce metamers for three channels. Mostly false: three channels discriminate essentially all natural reflectances, with only artificial or theoretical exceptions. False: the evidence shows three channels suffice to tell apart all natural surface reflectances.

Set before the research began.

1. E1 Frequency of Metamerism in Natural Scenes PubMed Central (NCBI); journal name not shown in fetched text · 1 August 2003

   “Metamers arise because the number of degrees of freedom in the sensor system, three for the cones in the normal human eye or a typical camera, is smaller than the number of degrees of freedom needed to specify different spectra.”

2. E2 Frequency of Metamerism in Natural Scenes PubMed Central (NCBI); journal name not shown in fetched text · 1 August 2003

   “The relative frequency of metameric pairs of surfaces, expressed as a proportion of all pairs of surfaces in a scene, was very low.”

3. E3 Frequency of Metamerism in Natural Scenes PubMed Central (NCBI); journal name not shown in fetched text · 1 August 2003

   “This is because the probability of two randomly chosen natural reflectance spectra producing numerically equal tristimulus values under a particular daylight illuminant is, in principle, vanishingly small. None were found with the present data set.”

4. E4 Frequency of Metamerism in Natural Scenes PubMed Central (NCBI); journal name not shown in fetched text · 1 August 2003

   “There has been some speculation that they are rare7-9; yet despite a large literature on metamerism (reviewed in Refs. 1 and 2), particularly concerning theoretical issues,10-12 few data are available on the actual frequency of metamers in natural scenes.”

E1, an independent colour-science paper, says metamers arise because a three-channel sensor such as the human cones or a typical camera has fewer degrees of freedom than are needed to specify different spectra. So in principle three channels cannot tell apart every reflectance. The same study reports that metameric pairs of natural surfaces occurred at a very low relative frequency (E2), and it found no pairs with exactly equal tristimulus values (E3). Earlier work had suggested natural metamers are rare (E4). This matches the 'mostly true' criterion: the claim holds in principle and is accurate as stated, but natural metamers are rare.