Colour is normally regarded as a property of light, its wavelength. In such a definition, micro waves or FM broadcasting signals would also be colours since the only difference with light is its wavelength. Visible light goes from 380 nm to 780 nm whereas FM broadcasting is in the range of meters. In reality we only know about visible light colour, that is the only one our retina is capable of processing.

So there’s no such a thing as colour “out there”. The experience that we call colour can be relatively unrelated to the properties of light, depending on what else the brain is experiencing at the same time or even what’s it expecting to see.

We are used to the basic names of colours like red, green, blue, etc. We even speak about “greenish yellow” but that’s not so in all cultures. A study conducted by Berlin and Kay* (1969) on more than 100 languages of different cultures showed that the primary colour names are considerably consistent across cultures although different cultures have a different number of them. Although the study and its further continuations have been a matter of a certain controversy it’s worth considering it here. (See Paul Kay's personal page) 

The naming of colours is consistent across cultures, but not all of them have the same number of colours. In those that have only two names they always correspond to black and white, the third is always red and then come green and yellow or yellow and green. Drawing by tha author inspired in the one in Colin Ware's book  Information Visualisation.

In those languages that only have two words for colours, they are always white and black. Of those that have more names for colours, the third is always red. The fourth and fifth are either green and yellow or yellow and green. The sixth is blue, the seventh is brown and only then come, grey, violet and other colours in no special order. 

The important thing is that this ordering of the six primary colours coincides with the colour opponent theory of colour that establishes that, from the perceptual standpoint, there are six primary colours arranged as opponent pairs along three axes. These three axes correspond to the three channels that can be obtained combining the stimulus of red, green and blue cone cells in the retina.

• Luminance channel. It'is based on the addition of the inputs of the three types of cones and gives the intensity of light. This makes the black and white channel.
  
  
• Red-green channel, obtained by the difference in excitation of the green and red cones.
  
  
• Yellow-blue channel obtained by the difference between the blue cones and the sum of the red and green cones excitation.

Opponent colour theory. The addition of the signals of the three types of cones gives the luminance channel, Black and White (B-W). The difference between Green and Red gives the Red-Green channel (R-G) and the difference between the blue signal and the composition of the other two gives the Yellow-Blue channel (Y-B). Drawing by the author.

There is physiological evidence0 given by deValois** in cells of the brain of primates that imply that even though the cells in the retina obey the trichromacy theory, those in the visual cortex obey the opponent colours theory. The naming of colours also abounds in this direction.

English speaking people use eight categories to refer to colour (red, blue, green, pink, purple, orange, yellow and brown). Jules Davidoff of Goldsmith’s College of London found that the Berinmo tribe of Papua New Guinea has only five names for the same range of colours. Apparently there’s not only that the Berinmo make rougher divisions but that they experience different things than English speaking people when they look at the same colour. 

This could mean that having a language based concept maybe necessary in order to distinguish two colour categories. Another explanation states that since the Berinmo language has not developed concepts to make finer distinctions between colours, although their neurons are able to detect the difference, it does not reach higher cognitive levels since there isn’t a concept mapping for it that could reach the conscious level. 

That colour is a variable perception can be easily seen looking at some simple visual illusions (see the enclosed figures). 

• Herman's Grid: A grid of black squares on a white background that produces "phantom" grey blobs at the corners between them. They are produced by the way the electrical signals of the retina's photoreceptors process off-center stimuli. 
  
  
• Simultaneous brightness contrast. If we put strips of homogeneous grey colour on top of a black background that degrades progressively into white, the color we perceive in the strips depends on their position in the varying colour background. 
  
  
• Colour contrast. The same colour on different background can be seen as different and vice versa, different colours can be perceived as very similar if we put the on top of different backgrounds.

Herman's Grid. In the intersections we are not looking at directly appear grey dots that disappear when we try to look at them.	Simultaneous brightness contrast. The widest strip and the six squares of the second row are all of the same hue of uniform grey. The six square in the last row have darker grey but in all of them is the same.
Colours perceived as similar when in reality are noticeably different, due to the context in which they are placed.	Colour contrast The pink crosses have exactly the same colour, although they appear to be different on different background.

There are many other examples, but the important thing is realising that perception is not a monolithic issue. Ultimately it depends on the specific person our work tries to address. 

For this reason the unconscious but otherwise usual approach to interface and web design of considering that everybody perceives and even behaves in the same way [as the designer] leads to a uniformity and lack of innovation we have to avoid. An interdisciplinary approach is required in these days of homogeneity. 

* Berlin, B., and Kay, P. Basic color terms: Their universality and evolution. Berkeley and Los Angeles: University of California Press, 1969.

** DeValois RL, Abramov I, Jacob GH (1966) Analysis of response patterns of LGN cells. J Opt Soc Amer 56: 966-977.  

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