Purple is considered the most difficult color for the human eye to distinguish. There are several reasons why our eyes struggle to see shades of purple clearly.
The physics of purple light
The visible light spectrum ranges from violet at around 380 nanometers to red at around 740 nanometers. Purple is a non-spectral color, meaning there is no single wavelength of light that corresponds to purple. Instead, our eyes perceive purple when they receive a combination of short and long wavelengths, specifically red and blue light.
Red light has the longest wavelength that humans can see, at around 700nm, while blue has a shorter wavelength of around 450nm. When these two wavelengths are mixed together in similar concentrations, our eyes and brain perceive the color purple. Because purple does not correspond to a specific wavelength, it is more difficult for our visual system to focus on it compared to other colors like green or orange that have their own wavelengths.
Sparse representation in nature
Another reason purple is challenging to see is its relative rarity in nature. Our visual systems evolved to be highly attuned to colors that were most abundant in our natural environment. Shades of green, brown, and blue are prevalent across landscapes and organisms. Purple, on the other hand, is less ubiquitously present. There are relatively few naturally occurring purple pigments and compounds compared to other colors.
This means our eyes and brains have less practice distinguishing different shades of purple. The neural pathways dedicated to purple perception are less developed relative to other colors. With less representation of purple on the natural world, our visual systems are less specialized to recognize variations in purple hues.
Receptor biology
At a biological level, the cone photoreceptor cells in our retinas also contribute to the difficulty of seeing purple. Human eyes have three types of cones: S cones, M cones, and L cones. The S cones are most sensitive to short, blue wavelengths of light. The M and L cones detect middle and long wavelengths like green and red, respectively.
Notably, we have very few S cones relative to M and L cones. S cones make up just 5-10% of our total cones. This means we have less photoreceptor coverage for the short wavelengths that make up one half of purple light. With fewer specialized cones for detecting blue light, discriminating purple hues becomes more challenging.
| Cone Type | Peak Sensitivity | Percentage of Cones |
|---|---|---|
| S cones (blue) | 420-440nm | 5-10% |
| M cones (green) | 534-545nm | 30% |
| L cones (red) | 564-580nm | 60-65% |
Neural processing
Additional aspects of visual neural processing make distinguishing purples more difficult. Firstly, the signals from S cones are overall weaker than those from M and L cones. This means the blue light input is less amplified on its way to the brain’s visual cortex.
Furthermore, the blue and red inputs remain relatively segregated through the initial retinal pathways. They are not fully combined into an integrated purple perception until higher levels of visual processing. This delay in mixing the signals likely hinders fine purple discrimination.
Short wavelength focus
The short wavelengths of blue light that contribute to purple also bring additional challenges. Shorter wavelengths are more prone to scattering as light passes through the eye. This causes slight blurring and difficulty with focus. As a result, images heavy in blue/violet light can appear slightly out of focus compared to long wavelength dominant images. This makes distinguishing fine variations in purple more difficult.
Low purple saturation
Most purples we encounter in everyday life are desaturated or diluted versions of the color. Vibrant, fully saturated purples are relatively uncommon. Desaturated colors are harder to distinguish in general, and this effect is exaggerated for purple due to the factors above. The subtle variances in less saturated purples are more difficult for our visual systems to detect.
Rods at night
In low light conditions, our eyes rely more on rod photoreceptors than cones for vision. Rods are worse than cones at detecting color variations. At nighttime, differences between purple hues are even harder to discern due to this rod takeover.
Age-related effects
As we age, the lenses in our eyes increasingly yellow and the pupils shrink in size. These effects reduce the amount of short wavelength blue light reaching the retina. This disproportionately impairs purple perception more than other colors. The elderly end up with a form of acquired purple blindness.
Visual cortex
The final stage of color perception arises in the brain’s visual cortex. Interestingly, neuroimaging studies show the color area of the visual cortex responds more weakly overall to purple than other major colors like red and green. The cortical circuits dedicating to processing color appear less specialized for purple compared to other hues.
Difficulty finding purple objects
In visual search tasks, subjects take longer to locate purple objects compared to equally bright objects of other colors. This indicates purple items do not tend to automatically ‘pop out’ of backgrounds. Instead, intentionally scanning for purple within a scene is more effortful.
No purple cones
While some animals like birds and fish have tetrachromatic vision with four cone types, humans are trichromatic and lack dedicated purple photoreceptors. This likely contributes to our difficulty perceiving purples relative to species with an extra retinal channel devoted to this hue.
Cultural associations
Across cultures, purple is associated with extravagance, creativity, magic, and mystery. These associations arise partly from the rarity of purple in nature. Before synthetic pigments, obtaining saturated purple dyes was expensive and labor-intensive. The difficulty of producing purple colored our psychological perception of the color.
Confusion with blue
When asked to select colors from an array, people make more errors when identifying purples compared to other chromatic colors. Purple is most commonly confused with blue. This reflects the poor perceptual segmentation of blue-violet wavelengths into distinct categories.
Purple vs. blue confusion matrix
| Blue reported | Purple reported | |
|---|---|---|
| Blue shown | 95% | 5% |
| Purple shown | 20% | 80% |
This table shows results from a color perception experiment where participants had to label colors. It illustrates people were more likely to mislabel purples as blue, than vice versa.
Optical illusions
Several optical illusions take advantage of the visual system’s poor purple discrimination. In “purple fringing”, blurring purple edges into nearby colors creates striking chromatic fringes. The Shevell disk illusion arranging purple spots to appear non-uniform exploits the purple-blue confusion.
Conclusion
In summary, purple’s status as a non-spectral color, its rarity in nature, the lack of purple retinal receptors, insufficient cortical representation, poor focus on short wavelengths, low purple saturation in real objects, age-related effects, and tendency to be confused with blue all contribute to purple being the most difficult hue for humans to distinguish. Our vision has limited sensitivity for discerning variations in this elusive color blend.