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I know that white colour is perceived on excitation of red, blue and green cones simultaneously.

It is also said that a colour-blind person doesn't see a particular colour and just perceives it as a shade of grey.

If their (for example all red) cones are affected, then how do they perceive white colour?

Clarification: Will a person with defect in red cones perceive white colour when green and blue colour only fall on his retina?

So, basically he will detect RBG and BG both as white.

If not then how will he be able to differentiate?

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    $\begingroup$ "People with deuteranomaly and protanomaly are collectively known as red-green colour blind and they generally have difficulty distinguishing between reds, greens, browns and oranges. ... To these people the world appears as generally red, pink, black,white, grey and turquoise." Depends on what type of color blind. colourblindawareness.org/colour-blindness/… $\endgroup$
    – Krythic
    Commented May 4, 2017 at 4:31
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    $\begingroup$ If red is absent then how is white colour perceived? $\endgroup$
    – JM97
    Commented May 4, 2017 at 4:35
  • $\begingroup$ @krythic Why don't you write this as an answer? $\endgroup$
    – Arsak
    Commented May 4, 2017 at 5:14
  • $\begingroup$ @Marzipanherz I don't care about this branch of stack exchange. Why don't you do it? Free points. $\endgroup$
    – Krythic
    Commented May 4, 2017 at 5:16
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    $\begingroup$ as long as they have rods black white and greyscale will always be possible. $\endgroup$
    – John
    Commented May 4, 2017 at 15:00

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It's definitely not white or grey generally, it's a mix of other colors, they often have low resolution of a particular color so that it's less visible rather than visible and grey.

enter image description here

Here's a page where you can mouse over a color wheel and see a version in color blind mode: http://www.archimedes-lab.org/colorblindnesstest.html

enter image description here

The first type of cone is primarily sensitive to short wavelengths (blue), another to medium wavelengths (green) and one to long wavelengths (yellow). The yellow cone is usually referred to as the red cone. While its sensitivity peak lies in the yellow wavelength band, it is also quite sensitive to red. A single cone cannot detect color, as it provides only a scalar number indicating the total light energy it absorbs. For example, the red cone by itself cannot distinguish red from yellow, green or orange. Red is detected by a combination of high activation of the red cone, low activation of the green cone and no activation of the blue cone. This page has a very comprehensive graphs and a first hand account of Tritanomaly

Evolutionary gene mechanisms have a particular knack of varying vitally important organs and functions, arm length, walking gait, hair type, color, and colorblindness is perhaps an expression of high variance to a survival critical and modular organ which animals have a high variance in depending on their habitat and survival requirements.

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    $\begingroup$ Thanks. One last query: consider a person who has defect in red cones, now if we put Blue and green light on his retina will he detect it as white? Since red cones are anyhow not functioning in his eyes , will he perceive it as white? If not then by what mechanism will he perceive white colour? $\endgroup$
    – JM97
    Commented May 4, 2017 at 11:04
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    $\begingroup$ when all the cones are signalling a balanced range across the spectrum he will see it as white. I changed the answer with an extra graph to explain that the cones are not RGB, they have broad reception, with a page to a guy who says how he got used to greens being grey or white and reds being confused with yellow and orange. None of the cones sense precisely red so there won't be a very narrow reaction to red colors. $\endgroup$
    – bandybabboon
    Commented May 4, 2017 at 14:39
  • $\begingroup$ Well, we can't see things as colorblind can (I have very strong reason to think even non-colorblind people don't see colors identically due to veriations between cone cells sensibility). Given page does not give us their perception. Colorblind people can see some impossible colors, for example. For us it is a mystery until a device is made capable of affecting cone cells selectively. And I'm not sure achromatopsia makes people see everything grey. Grey is a composition of all three types of cones being involved. But their cones, I guess, don't work. $\endgroup$
    – rus9384
    Commented Aug 31, 2018 at 23:24

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