What are perceptual colors?

As far as sounds are concerned, there is an area relating to art and culture: music. When we use the vocabulary of this domain we know without ambiguity that we are on the side of the subjective perception of sounds.

In the field of colors, the use of the word “color” does not make it possible to make such a clear distinction from the context in which we are situated. A physicist will use this word as frequently as a painter or designer. The word “color” can either refer to the physical phenomenon or to its perception for the human being. the expression “perceptual colors” makes it possible to remove this ambiguity and to locate clearly on the side of subjective perception.

When we speak of perceptual colors, it is therefore not a subset of the color spectrum or colors with particular characteristics. It is simply a matter of immediately showing that we are in the realm of human perception.

How does the distinction between “colors” and “perceptual colors” work in practice? Strictly speaking, these are the same colors. What changes is their organization in a classification system and more precisely the nature of the distance that is used to build this classification system. Color classification systems generally identify colors by their coordinates in a three-dimensional coordinate system. The different colors identified are then considered as equidistant from each other. The nature of the distance varies depending on the tool used to measure the difference between two point colors. We can use physical measurement instruments or our subjective evaluation, our “eye”.

To make this more concrete let us consider, in the IT field, two different color systems: the sRGB system and the CIE LAB system :

In the RGB triangle, the distance measurement used is based on the difference of the electrical signals sent to the phosphors on the screen. In the CIE LAB triangle, this measurement is based on the subjective appreciation by a sample of human observers of the difference between two test colors. Although there is a certain logic of color arrangement in the RGB triangle, the organization of the colors of the CIE LAB triangle should seem more “natural” to an observer with ordinary vision.

We perceive the difference in color organization in the juxtaposition of graphic representations of a perceptual color system (CIELCH) and a material color system (sRGB). For both views, the hue is on the abscissa, the saturation on the ordinate. The value is constant at the average system level.

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