Answer · confidence: partial
Why might two people see the same sky in different shades of blue?
Research question: How do the cone photoreceptors in the human eye, opponent-color processing, and the brain turn a mix of scattered wavelengths into the perception of blue, and how do metamerism and differences among observers mean that two people can see the same sky in slightly different ways?
The records show that Rayleigh scattering makes the sky look blue because blue light scatters about 10 times more strongly than red light in air 23, and the scattered light reaches the eye from every direction 6. Hering proposed the opponent-color idea before cone photopigments were known, and he believed the antagonistic mechanisms were neural 710. The facts do not explain cone transduction or how the brain builds blue perception, though Hering did study metameric mixtures and the influence of macular pigment 13. Observers' cone responses can differ, since individual cone sensitivities can be inferred from each person's color matching functions 8, and older observers show more variability in color matching 12. Standard mean parameters may not apply to all observers 15; the newest of these facts dates to October 2024, about two years before today 911.
Checked 2026-10-18. Only documented and supported facts are used.
Dig deeper on this questionSources
[1] supported
The CIE roadmap explicitly addresses how normal variations in cone fundamentals, due to age, field of view, and individual diversity, affect colorimetry.
"with explicit consideration of the impacts of normal variations of the cone fundamentals due to age, field of view, and individual diversity."CIE 254:2024 A roadmap toward basing CIE colorimetry on cone fundamentals
[2] supported
Blue light is scattered in air about 10 times more strongly than longer-wavelength red light, because the scattering structures (local density fluctuations of the air) are very small-scale; this is Rayleigh scattering.
"Licht mit blauer Spektralfarbe wird in Luft etwa 10-mal stärker gestreut als das langwelligere rote Licht, weil die streuende Struktur – lokale Dichteschwankungen der Luft – sehr kleinskalig ist, siehe Rayleigh-Streuung ."Himmel (planetar)
[3] supported
Rayleigh scattering explains why the sky looks blue.
"Die Rayleigh-Streuung erklärt, warum der Himmel blau erscheint."Rayleigh-Streuung
[4] supported
Blue light has a higher frequency than red light, so it is scattered more strongly.
"als rotes und wird daher stärker gestreut."Rayleigh-Streuung
[5] supported
Because blue light is scattered out of direct sunlight at midday, the sun looks yellow.
"Deshalb erscheint die Sonne gelb."Rayleigh-Streuung
[6] supported
The combined scattered light from all directions makes the sky look blue from every direction.
"Die Summe allen Streulichtes lässt den Himmel aus allen anderen Richtungen blau erscheinen."Rayleigh-Streuung
[7] documented
The opponent-process idea of color perception was developed by E. Hering before cone receptors or cone photopigments were known.
"The idea of opponent or antagonistic processes as the basic physiological substrate of our ability to perceive colors was originally developed by E. Hering before the discovery of what was initially called visual purple, and long before anything at all was known about cone receptors or the cone photopigments."Opponent-Colours Theory in the Light of Physiological Findings (1985)
[8] supported
Individual cone sensitivities can be inferred from each person's individual color matching functions, which is a classic problem in color vision, implying that observers' cone responses can differ.
"Estimating individual cone sensitivities on the basis of their corresponding individual color matching functions is a classic problem of color vision."Estimation of an individual's Human Cone Fundamentals from their Color Matching Functions (2010)
[9] documented
The study found that the chromaticity domain enclosed by the spectrum locus and purple line does not contain the chromaticity coordinates of all stimuli, and spectral chromaticity coordinates do not follow wavelength order along the spectrum locus.
"It is found that the domain 𝛺 does not contain the chromaticity coordinates of all stimuli, and spectral chromaticity coordinates do not distribute in the wavelength order along the spectrum locus, which results in no way to determine the Helmholtz coordinates for certain stimuli."CIE Colour Matching Functions and Cone Fundamentals: Problems and Modifications (2024)
[10] supported
Hering believed the antagonistic color mechanisms were neural and linked to opposite neurochemical processes.
"Hering’s antagonistic mechanisms were thought by him to be neural, and to be somehow associated with opposite neurochemical processes."Opponent-Colours Theory in the Light of Physiological Findings (1985)
[11] supported
It was previously assumed that the chromaticity diagrams based on the CIE 1931 or 1964 colour matching functions contained all possible chromaticity coordinates of stimuli.
"In the past it was generally accepted that the chromaticity diagram based on either 1931 or 1964 colour matching functions (CMFs), contains all the chromaticity coordinates of stimuli"CIE Colour Matching Functions and Cone Fundamentals: Problems and Modifications (2024)
[12] supported
Older observers showed more variability between individuals in colour matching, which helps explain why two people can match the same colour differently.
"The experimental results show relatively little variability with age, except as expected increases in lens density with age, and older observers showed more inter-observer variability."The Age Effect on Observer Colour Matching and Individual Colour Matching Functions (2023)
[13] supported
Hering published quantitative studies of metameric color mixture and considered effects such as intensity invariance of color matches and the influence of macular pigment.
"although he published quantitative studies of metameric color-mixture and was concerned with matters such as intensity invariance of color matches and the influence of macular pigment on such data"Opponent-Colours Theory in the Light of Physiological Findings (1985)
[14] supported
Individual observers' color matching data are tested under the assumption that they share a common photopigment spectral absorptance, the Stockman and Sharpe 10 degree function.
"with the assumption that they have the Stockman and Sharpe 10 degree photo-pigment spectral absorbtance in common."Estimation of an individual's Human Cone Fundamentals from their Color Matching Functions (2010)
[15] supported
Standard mean colour matching parameters assumed by the CIE, such as macular pigment optical density, may not apply to these observers, indicating that individual differences matter.
"The analysis suggests that some of the mean colour matching parameters assumed by the CIE, such as the macular pigment optical density, do not apply for these observers."The Age Effect on Observer Colour Matching and Individual Colour Matching Functions (2023)