I understand that dichromats have one of their cone missing/not functioning. And as for Monochromats, 2 or all of their 3 cones are missing/not functioning. And I read from Wikipedia - Color Blindness :

Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.


Monochromacy occurs when two or all three of the cone pigments are missing and color and lightness vision is reduced to one dimension.

Since in the description they relate the cones with Red, Blue, Green (RGB) a lot, eventhough I'm fully aware that relation of cones and RGB is most likely for historical reason . I'd like to assume in this reduction from 3-dimensional into less dimensional colorspace dimension written in Wikipedia based on RGB colorspace.

This is original RGB colorspace in 3-Dimension

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Question 1 : How exactly reduction from 3-dimensional to 2-dimensional or even 1-dimensional in colorblind case happen ? For example in Protanopia, would they lose the Red axis ? So only Blue and Green axis left ? (Any reference will be appreciated).

I also read this question If people with colorblindness lack one type of cone cells, shouldn't they be unable to recognize one particular color?. And it's written that Dichromats can match any color they see with some mixture of just two primary colors. Cones do not literally interpret colors. The three cones work together to encode a signal.

Question 2 : If cones don't really interpret any color in human color vision system. Is it wise that losing cone means reduce from 3-dimensional color into 2-dimensional by completely erase the corresponding color axis to the cone ?


For Question 1 :

Taken from the book Visual Transduction and Non-Visual Light Perception (p308),

Univariance, Monochromacy, Dichromacy, and Trichromacy

  1. If only one photoreceptor type operates, vision is monochromatic or reduced to a single dimension

  2. If only two cone photoreceptors operate, vision is dichromatic or reduced to two dimensions


Most observers with normal color vision have three classes of cone photoreceptor and are therefore trichromatic.

So, I'm sure in my Question 1, they really refer to Young–Helmholtz Trichromatic theory. A theory when human color vision possess 3 types of cones thus they have three dimensions of color vision.

However, I still need some opinion related to Question 2. Any comments will be appreciated.


1 Answer 1


Let's start with a quick overview of the different types of dichromats in man:

  • Protanopes have no functional red cones. Red appears as black. Certain shades of orange, yellow, and green all appear as yellow.
  • Deuteranopes have no functional green cones. They tend to see reds as brownish-yellow and greens as beige.
  • Tritanopes have blue-yellow color blindness and lack blue cone cells. Blue appears green and yellow appears violet or light grey.

Now color vision can be modeled with the Hering model, where there are two opponent channels, namely red-green and blue-yellow. These opponent channels determine color vision. Because of these two limbs, color space is often represented as triangles, rather than cubes as in your post. Fig. 1 shows the color space in trichromats. In Fig. 2 the same is shown, alongside the three types of dichromatic deficiencies.

Fig. 1. Color space in trichromats. source: Lighting Analysis

Fig. 2. Triangles in trichromatic and dichromatic vision. source: Chartlr.

  • $\begingroup$ Thanks! I'm aware of this triangle 2D representation. I believe this is adaptation of colorblindness taken from CIE 1931 color space. I believe color vision can also be represented by Helmholtz trichromatic theory which has 3 primary color, exactly like RGB. What I want to confirm is the reduction of 3-dimensional space into 2 (or 1) dimensional space in colorblind. I'm open to any possibility that the RGB 3D diagram above is wrong. I'm not sure is it RGB or which other 'color dimension' they meant, since it's not exactly written there. $\endgroup$
    – raisa_
    May 2, 2018 at 7:00
  • $\begingroup$ It depends on the definition of dimension, but yes, you could say you loose one. $\endgroup$
    – AliceD
    May 2, 2018 at 7:17

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