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Number of Colors Distinguishable by the Human Eye

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Calkins, David J. Mapping color perception to a physiological substrate. The Visual Neurosciences Volumes 1 and 2 [institutional subscription required]. The MIT Press, 1993. "The tremendous variability in the spectral composition of light reflected from surfaces lends itself to eliciting a daunting gamut of more than 100,000 discriminable colors, and the variation in the names we assign these colors is limited only by scope of human experience." 100,000
Wyszecki, Gunter. Color. Chicago: World Book Inc, 2006: 824. "Experts estimate that we can distinguish perhaps as many as 10 million colors." 10 million
Kleiner, Kurt. What we gave up for colour vision. "New Scientist." January 24, 2004: 12. "Humans, other apes, and Old World monkeys have trichromatic vision, with eyes containing three colour receptors, sensitive to blue, green, and yellow-red. They allow us and our Old World relatives to distinguish around 2.3 million colours." 2.3 million
Myers, David G. Psychology. Michigan: Worth Publishers, 1995: 165. "Our difference threshold for colors is so low that we can discriminate some 7 million different color variations (Geldard, 1972)." 7 million

Color is often mistaken as a property of light when it really is a property of the brain. Our experience of color depends not only on the wavelength of the light rays that hit the retina, but also the context in which we perceive it- things such as background colors, lighting, familiarity, and surroundings.

Within the retina are buried receptor cells called rods and cones. When light energy strikes them, neural signals are created as a result of chemical changes. The signals are then routed through neighboring bipolar and ganglion cells that form the optic nerve. This nerve then transmits information to the brain's visual cortex. Our 120 million rods are responsible for our perception of black, white, and gray. They are the most sensitive in dim light. Our 6 million cones, on the other hand, are what enable us to see color and fine detail. They function in well-lit conditions and become ineffective with diminished illumination.

Source: How We See: The First Steps of Human Vision

There are three primary colors- red, blue, and green- that make the millions of colors that are distinguishable by the "normal" human eye. Each eye contains three receptors (one for each primary color) that generate the experience of color when stimulated in various combinations. This is known as the Young-Helmholtz Trichromatic Theory. Those who have defective cones have difficulty seeing certain colors and are known to be color-deficient. With this in mind, it is fair to then say that the number of colors the human eye can discriminate depends mainly on the sensitivity of the individual's eyes.

Visible light is an electromagnetic wave that has a wavelength range of approximately 380 nanometers to 740 nanometers (refer to the table below).

Source: Wikipedia: Color
Color Wavelength Interval Frequency Interval
red ~ 625–740 nm ~ 480–405 THz
orange ~ 590–625 nm ~ 510–480 THz
yellow ~ 565–590 nm ~ 530–510 THz
green ~ 500–565 nm ~ 600–530 THz
cyan ~ 485–500 nm ~ 620–600 THz
blue ~ 440–485 nm ~ 680–620 THz
violet ~ 380–440 nm ~ 790–680 THz

The monochromatic colors of the rainbow (red, orange, yellow, green, blue, cyan, and violet) have their characteristic wavelengths within this range. When light strikes an object, it can be absorbed, reflected, or scattered. When the surface absorbs all wavelengths equally, we perceive it as black. When the surface reflects all wavelengths equally, we perceive it as white.

We often use the HSB model to classify colors. This model includes three components: Hue, Saturation, and Brightness. The hue of a color can be referred to as a particular shade or appearance of a color. There are 150 hues the eye can distinguish and they include the colors of the visible light spectrum. Brightness refers to the amount of light emitted by an object. Saturation is the purity of a color, or the intensity of a hue. A less saturated color would be more dull, while a highly saturated color would be more vivid. The graphic below displays the saturation levels of the color red, where the bottom has the least saturation.

Source: Wikipedia: Saturation (Color Theory)

Jennifer Leong -- 2006