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Quora explains how white and black colors fit into the spectrum of visible light. It explains that white is all colors together while black is the lack of color.

So, where is Gray? Gray is the mix of all-colors and no-color!! What does that mean? Can somebody explain Gray, the mysterious color?

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All colors are only in the mind. Light has a mix of wavelengths, but it doesn't have color until someone sees it.

When light enters the eye, it hits rods and cones in the retina. Cones are color receptors. There are three kinds. Each kind is sensitive to a range of wavelengths. Color is the result of stimulation of the cones, and additional processing in the brain.

enter image description here

The image is from The Color-Sensitive Cones at HyperPhysics. Copyright by C. R. Nave, Georgia State University. A good starting link is the Light and Vision page.

Loosely, the sensors are sensitive to long, medium, and short wavelengths. The ranges overlap. Most light, even single wavelength laser light, stimulates more than one. The graph shows which are stimulated by single wavelength light at different wavelengths. The colors we see are determined by the mix of stimulations. The bottom of the graph gives names of colors for single wavelength light.

Grey is not on the list. Grey requires a mix of wavelengths that stimulate the three types more or less equally. So do black (very little stimulation) and white (more).

There is more to it than that. The perception of color is affected by colors around it. There are photographs where two different patches reflect the same light. But the colors we perceive are different because of the surroundings. For example, see the Checker Shadow illusion.

enter image description here

By Original by Edward H. Adelson, this file by Gustavb [Copyrighted free use], via Wikimedia Commons

Also no wavelength will stimulate only the "Green" cones. They are always stimulated in combination with other cones. I once read it is possible to stimulate them with a probe. The person saw a color he had never seen before. I wish I could find a link. Quora might be a good place to start.

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    $\begingroup$ The checkerboard contrast illusion is particularly revealing on this matter. Our brains do so much post-processing on the raw visual data that you can't reliably deduce any absolutes from what you "see". $\endgroup$ Jun 13 '17 at 16:53
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    $\begingroup$ Similarly there is no "brown" on the spectrum. That's a psychological response to seeing certain colors near to each other. The graphic also mislabels "purple". Where it says purple, it should say violet. Purple occurs when the red and the blue cones are stimulated nearly equally and also does not correspond to any single wavelength of light. $\endgroup$
    – Brick
    Feb 19 '20 at 19:58
  • $\begingroup$ @Brick - You are right about purple. I had read that brown is dark yellow. Blue and dark blue look alike. Likewise red and dark red. But dark yellow is a completely different color. $\endgroup$
    – mmesser314
    Feb 19 '20 at 20:28
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All colors are combinations of three wavelengths of visible light, red, green and blue.

  • White = All Red + All Green + All Blue
  • Black = No Red + No Green + No Blue
  • Grey = unsurprisingly Half Red + Half Green + Half Blue!

Read this Wikipedia Page: https://en.wikipedia.org/wiki/RGB_color_model

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    $\begingroup$ Sensation of color can be obtained by combinations of 3 wavelengths but this does not mean that the colors "are" necessarily combinations. You can get the sensation of red with one single wavelength (actually relatively narrow interval). The spectral composition of white, gray and black are the same, relatively flat over the visible range. The difference is just a matter of brightness. $\endgroup$
    – nasu
    Jun 13 '17 at 14:28
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    $\begingroup$ @nasu One of the things that color and contrast illusions demonstrate is that the qualia of color is far more complicated than just the response of retinal sensory cells: it depends on what is happening in other parts of the visual field and even on other recent observations. $\endgroup$ Jun 13 '17 at 18:06
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    $\begingroup$ This seems to be a bit incomplete. Just display a gradient on your monitor and then change its brightness. You still have a black to white gradient, the grey does not turn white but is a brighter grey. So it is half in your RGB color model, but this seems not to correspond directly to less bright red/green/blue light, there you seem to only be able to change the color ratios. $\endgroup$
    – allo
    May 28 '18 at 9:53

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