The Journal for Weavers Spinners and Dyers

The Theory of Colour Perception

Sarah A. Pape, Tynedale and Durham Guilds

Introduction

If an object is described as being sky blue, each of us will visualise a colour that matches our understanding and recollection of the natural colour of the sky. However, the sky can be many different shades of blue according to the time of day, season of the year, weather conditions and geographical location. If a group of people stood side by side at the same location staring at the same piece of sky and tried to match its colour to a standard chart, the likelihood is that each would identify a slightly different shade of blue. This variation is because we all perceive and remember colour slightly differently. Colour perception depends not only on how light enters our eyes and triggers nerve impulses within the retina, but also on how our brains interpret these signals.

Is all light the same?

The eye is able to detect light with a range of wavelengths from around 450nm (violet) to around 700nm (red). Within this range lie all the colours of the rainbow, although the order is backwards compared with the way we were taught in school! When all these wavelengths are present in a beam of light we call it ‘white’ light. However, there is often a preponderance of one particular wavelength, such as blue (e.g. twilight or fluorescent lights) or yellow (e.g. midday sun or halogen bulbs).

Variations in the composition of white light alter the pattern of reflection from objects, producing subtle differences in the nerve impulses that reach the brain. This accounts for that familiar problem of choosing clothes in a shop that is illuminated by very bright fluorescent lights but finding that the colours are not as you remembered them when you wear the outfit in daylight. Fashion editors will advise you to take the garment to the door of the store so that you can make a better assessment of the colour in daylight, although you may find that this produces unwanted attention from the security staff! The same goes for buying yarn in a shop or dyeing fibre indoors.

How do we see colour?

Don’t worry. I am not going to give a detailed explanation of the anatomy and physiology of the eye because that is not necessary in order to understand how we perceive colours. However, it will help to understand the basics. To see an object it needs to be illuminated. Light is a form of radiation, like radio waves and microwaves. Visible light travels through the clear structures at the front of the eye to reach the retina at the back of the eye. Here, the light energy activates specialist nerve endings, which send nerve impulses via the optic nerves to the brain, producing a visual image of the object. If the object is only dimly lit we perceive an image in shades of grey, like a black and white photograph. This is because only the more sensitive nerve endings (the rods) are activated, which are unable to detect differences in colour. In brighter light a second set of nerve endings (the cones) are activated. There are three different types of cone cells within the normal retina. Each type is activated by a range of colours but is more responsive to one colour or another (Fig 1). The combination of responses produced by the three different types of cone cells produces a pattern of nerve impulses that is sent to the brain. The brain processes this information and produces our awareness of colour. When light falls onto an object it will be either absorbed or reflected by its surface. Only the reflected light enters the eye. If the object absorbs those rays of light with short and medium wavelengths, only rays with longer wavelength will be reflected and enter the eye. This activates one set of cones more than the other two, producing a pattern of nerve impulses that the brain interprets as the colour red. Similarly, if only the shorter wavelengths are reflected and enter the eye, a separate set of cones is stimulated, and the brain interprets this as violet. If rays of light across all the wavelengths of visible light are reflected from the object, all three types of cone are stimulated and we see the colour white.

Is all light the same?

The eye is able to detect light with a range of wavelengths from around 450nm (violet) to around 700nm (red). Within this range lie all the colours of the rainbow, although the order is backwards compared with the way we were taught in school! When all these wavelengths are present in a beam of light we call it ‘white’ light. However, there is often a preponderance of one particular wavelength, such as blue (e.g. twilight or fluorescent lights) or yellow (e.g. midday sun or halogen bulbs). Variations in the composition of white light alter the pattern of reflection from objects, producing subtle differences in the nerve impulses that reach the brain. This accounts for that familiar problem of choosing clothes in a shop that is illuminated by very bright fluorescent lights but finding that the colours are not as you remembered them when you wear the outfit in daylight. Fashion editors will advise you to take the garment to the door of the store so that you can make a better assessment of the colour in daylight, although you may find that this produces unwanted attention from the security staff! The same goes for buying yarn in a shop or dyeing fibre indoors. s

Colour blindness

Very few people have true colour blindness (i.e. the inability to see any colour at all). However, it has been estimated that approximately 1 in 12 men and 1 in 200 women have difficulty distinguishing between some colours. This tendency often runs in families, being carried on the X chromosome, but its severity is highly variable. Although we don’t fully understand the underlying basis of colour blindness a reasonable explanation is that one or other type of cone is defective. For example, poor function of the cones that are most sensitive to the longer wavelengths of light mean that the colour red is not perceived by the brain (a condition called protanopia). This applies not only to objects that reflect pure red light but also to those whose colour includes a red hue. For instance, a purple T-shirt will appear to be the same colour as a blue T-shirt because the colour-blind individual cannot detect the red element of the purple.

Fig 1. There are three types of cones in the retina. The first set are most responsive to the shorter wavelengths of visible light (violet, blue). The second set are most responsive to the medium wavelength of visible light (green, yellow). The third set are more responsive to the longer wavelengths of visible light (orange, red). The actual colour perceived by the brain depends upon the pattern of nerve impulses fired by all of the cones.
Fig 1. There are three types of cones in the retina. The first set are most responsive to the shorter wavelengths of visible light (violet, blue). The second set are most responsive to the medium wavelength of visible light (green, yellow). The third set are more responsive to the longer wavelengths of visible light (orange, red). The actual colour perceived by the brain depends upon the pattern of nerve impulses fired by all of the cones.

It is also reasonable to suppose that we are all different in our ability to perceive subtle gradations of colour. For instance, one person might see turquoise as a shade of blue and another as a shade of green. Others (me included) struggle to detect a difference between dark navy and black. Even trained artists will disagree about the precise description of a colour. Hence, many individuals and manufacturers of pigments and dyes have attempted to find a scientifically rigorous method for classifying colour.

The three dimensions of colour

Individual colours can be identified by their hue (red, orange, yellow etc.), value (degree of lightness or darkness) and intensity (from the pure hue to a neutral tone). Let’s consider each of these in turn.

Hue refers to the quality that distinguishes one colour from another. Artists talk about the three primary colours (red, blue, yellow) which cannot be made by mixing any other combination of coloured paints. These primary colours can be mixed in pairs to create the secondary colours (purple, green and orange) and all the intermediate hues (Fig 2).

Fig 2. An artists’ colour wheel showing the three primary colours (red, blue, yellow) and the three secondary colours (purple, green, orange)
Fig 2. An artists’ colour wheel showing the three primary colours (red, blue, yellow) and the three secondary colours (purple, green, orange)

Value, tone or tonal are terms that refer to the range of any colour from very light to very dark. For example, mixing red paint with white or diluting red dye with water will create paler and paler shades of red that we call pink. On the other hand, mixing red paint or dye with black will create darker and darker shades of red that we call maroon. Some hues are inherently light (e.g. yellow) and some dark (e.g. purple).

Intensity, purity and saturation are terms that refer to the brightness or dullness of a colour. Mixing pigment of a pure hue with increasing amounts of the colour opposite to it on the colour wheel (Fig 2) will dull the colour to a neutral shade of mud or ‘hoover dust grey’ as my art tutor would call it. I am sure that many of us were experts at creating these shades in school art classes. I certainly was. The explanation is that as soon as you include all three of the primary colours in a mixture, even if one is present in only the smallest quantity, the overall effect will be to dull the predominant colour. So, mixing blue and yellow dyes will create a pure green but if you add even the slightest touch of red (or any intermediate hue that contains red, such as orange or purple) you will dull the green towards khaki.

Conclusion

I hope that this short account will give you sufficient information to understand how we see colour and how three elements (hue, value and intensity) can be used to describe any single colour. In a future article I will explain the practical application of colour theory to textile arts. In the meantime you may wish to refer to one of the books in the suggested reading list for a more detailed account of colour theory.

Further reading

Albers J. (1975) Interactions of Color. New Haven & London: Yale University Press

Varley H. (Ed) (1988) Colour. London: Marshall Editions Ltd.

Sarah A. Pape

Spinner, weaver and dyer.

cover

This article appears in edition #267 of the Journal for Weavers, Spinners and Dyers.

The Journal is published on behalf of the Association of Guilds of Spinners Weavers and Dyers. It covers a wide range of textile subjects, including articles on historic textile techniques and cutting edge modern design.

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