I am a graphic designer and I really want to have knowledge about human colour perception but the longer I am looking for and the more I read the harder this topic seems to be. Physics is not my strong side so I need someone who check what I (think I) know so far and tell me if it's right or wrong, and answer my additional questions.

Below is what I think after reading many and many articles and seen some videos (sometimes with opposite answers or too far simplifies):

1. Nature of light

a) Ray of white light consists waves of different lengths.

b) Different wavelenghts give to the brain different impression of colour.

c) We can see it when a ray of light splits to the constituent waves in rain drops or when it go through material with higher density like glass prism. In those cases we can observe visible light spectrum which consists separate wavelenghts that give impressions of colours: violet, blue, cyan, green, yellow, orange, red.

2. Absorbtion and reflection

a) Surfaces of objects absorb and reflect some of lenghtwaves of white light ray.

b) Object that appears red absorb all the other wavelenghts and reflect only one wave from highlenghts range.

c) Object that appears green absorb all the other wavelenghts and reflect only one wave from medium range.

d) Object that appears blue absorb all the other wavelenghts and reflect only one wave from lowest range.

e) Object that appears yellow absorb all the other wavelenghts and reflect only one wave from range between medium and highest range.

f) Object that appears cyan absorb all the other wavelenghts and reflect only one wave from range between lowest and medium range.

g) Object that appears magenta absorb all medium wavelenghts and reflect simultaneously two waves from the lowest and the highest ranges. (← correct?)

h) Object that appears black absorb all wavelenghts and reflect no light.

i) Object that appears white absorb none of the wavelenghts and reflects all together (reflects whole white light ray).

3. Perception

a) In eye on the retina there are receptors called cones and rodes. Cones are responsible for colour interpretation.

b) There are three types of cones – receptors of small wavelengths [S] (mostly sensitive for ranges giving blue hues impressions), receptors of medium wavelenghts [M] (mostly sensitive for ranges giving green hues impressions), receptors of large wavelengths [L] (mostly sensitive for ranges giving red hues impressions but having small range giving violet hues impressions.)

c) Ranges of cones partially overlap.

d) Lightray, depending of it's composition after reflect from object, can stimulate one, two or all three cones.

e) Pure red/green/blue object reflect pure L/M/S wave and stimulate only one of cones at the time: red/green/blue.

f) Wavelenghts that lies between overlaping ranges of the cones stimulates them both eg. wavelenght responsible for yellow colour impression activate M ("green") and L ("red") cones, and wavelenght responsible for cyan colour impression activate S ("blue") and M ("green") cones.

g) Somehow magenta colour impression ("bright pink" or "intensive purple" as you can say, different from violet) appear when both S and L cones are activate by the one lenghtave from the lowest range and one wavelenght from the highest range. Magenta impression is not cause by one particular wavelenght that's why it's not appear in visible spectrum and rainbow. (← right?)

h) White impression appear when all three cones are stimulated with the same, high intensity.

i) Grey impression appear when all three cones are stimulated with the same, medium intensity.

j) Black impression appear when all three cones aren't stimulated (object absorb all the wavelenghts and doesn't reflect any of them to the eye).

k) Other colours that we can name such as brown, light pink etc. that we can't find in visible spectrum are actually variations of the colours include in spectrum but with different saturation and lightness. (← true?)

Please, If someone of you studied physics, optics, biology, knows the subject can help me to understand this I will be very gratefull. I am looking for these answers for a long time.

Let me know if this above is correct. Feel free to use simply examples and comparsions like you were explaining it to the child.

  • $\begingroup$ Color perception is even more complicated than that. The brain processes a whole scene. So we see "gray" in a BW photograph even when seen in colored light. See for example the debate about that dress. en.wikipedia.org/wiki/The_dress $\endgroup$
    – user137289
    Jan 6 '20 at 12:16
  • $\begingroup$ You need to read The Retinex Theory of Color Vision and later papers on the subject. $\endgroup$
    – Farcher
    Jan 6 '20 at 12:56
  • $\begingroup$ Unfortunately the blind watchmaker didn't design this system to be easy. The complexity you are encountering is because the system is complex. $\endgroup$ Jan 6 '20 at 18:36

You seem to have a decent understanding about the topic but there are a few mistakes. I shall only point out the points which are partially correct and incorrect.

i)From 2b to 2g- Objects don't reflect only a single wavelength but a range of wavelengths each with different intensities. A red shirt for example absorbs all non red wavelengths and reflects a range of red wavelengths (NOTE- the wavelengths reflected is a function of temperature). The wavelength(color) we end up seeing is the one reflected with maximum intensity. This applies to all colors. I'm not sure what 'range' you are referring to but each color has its corresponding wavelength in the visible spectrum.

ii) 3h, 3i and 3j- All shades of grey including white and black (achromatic colors) are process by the rod cells not the cones cells. The shade depends on the level of activation of the cells.

iii) 3g and 3k- All the colors visible to the human eye are present in the visible spectrum (hence the name). The 'adding' of colors is the work of our brain. Our brain has been hardwired to register a bit of green and a bit a red together as what we perceive as yellow color. Hence there are are two types of 'yellow'- the true yellow which is light in the 570nm to 590nm range and the false yellow which you see on a computer display. This applies to all the colors apart from those which form the peaks of the cone cells.

To summarize, our brain has mapped out a table where each unique combination of levels of activation of the three cone cells and the rod cells gives a different color which we end up 'seeing'. Also know that this combining of colors with light is known as subtractive color mixing. If you mix the same paints, you will get a different color because that happens through additive color mixing (red+green=brown and not yellow).

However, I am just a high school student and the actual mechanics of color perception are wayyy more complicated like Pieter commented. What I have mentioned are the basics which should be sufficient for understanding purposes. Please feel free to aske any questions in the comments :)

  • $\begingroup$ 3c,e,f: there is a lot more overlap in the sensitivity curves than the OP seems to be imagining; it is only at the extreme ends of the spectrum that a single pigment is excited; and pointedly wavelength that we would describe as "pure" colors excite multiple pigments. $\endgroup$ Jan 6 '20 at 18:34
  • $\begingroup$ Ah ... your paragraph (ii) is mostly wrong. The rods are saturated in bright light and don't contribute noticably to human vision under those conditions, but you can still see shades of gray. $\endgroup$ Jan 6 '20 at 20:01
  • $\begingroup$ The OP mentions in 3i and 3j that the light is not at maximum intensity which is when rods come into play. $\endgroup$
    – Sam
    Jan 6 '20 at 20:52
  • $\begingroup$ But what you wrote is wrong. You routinely see shade of gray when your rods are saturated. The behavior of the rods explains why you either don't see or barely see colors in very dim light. It doesn't explain why you can see gray in bright light. $\endgroup$ Jan 6 '20 at 20:55
  • $\begingroup$ @dmckee --- ex-moderator kitten♦ You seemed to be good informed. After reading my summary do you think I understand the subject well or is there something hardly wrong? I want to know it, and not only for me. I worked as a teacher in an art school and maybe someday I will want to do this again, and I want to be sure I won't say bullsh*t to my students when I will be explaining colour theory. $\endgroup$
    – Kubson
    Apr 19 '20 at 23:46

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