Tuesday, 29 May 2012

Colour theory - an overview

I thought at this point it was worth considering what makes colour and how we perceive colours.  Michael Freeman, in the OCA course notes, and in the additional material to be found in ‘Colour Theory’ on the OCA website, gives an overview of this but I wanted to explore things in greater depth.

Colour vision is the capacity to visualise objects based on the wavelengths (frequencies) of the light they reflect, emit, or transmit. Perception of colour by man is a subjective process whereby the visual cortex in the brain responds to stimuli produced when light enters the eye.  Hence, we all perceive colours differently to a degree.

The components of light have been known for several centuries. In 1670 Isaac Newton discovered that white light splits into its component colours when passed through a prism, but that if those individual spectra of colored light are subsequently passed through another prism and rejoined, they made a white beam. The characteristic colours are, from low to high frequency, red, orange, yellow, green, cyan, blue and violet. Sufficient differences in frequency give rise to a difference in perceived hue; the just noticeable difference in wavelength varies from about 1nm in the blue-green and yellow wavelengths, to 10nm and more in the red and blue. Though the eye can distinguish several hundred hues, when those pure spectral colors are mixed together or diluted with white light, the number of distinguishable chromaticities has been estimated to be up to ten million, although estimates and their scientific derivation vary according to source.

Colour is often mistaken as a property of light when it really is a property of the brain. Our experience of colour depends not only on the wavelength of the light rays that hit the photoreceptive retina, but also the context in which we perceive it. The background, lighting, familiarity of the situation, and surroundings of an individual can all impact on the precise colour that the brain interprets from the visual information it receives.

Within the retina are receptor cells called rods and cones. When light energy strikes them, neural signals are created as a result of chemical changes. The signals are then routed through neighbouring bipolar and ganglion cells that form the optic nerve which transmits information to the brain's visual cortex. 120 million rods are responsible for human perception of black, white, and grey and are the most sensitive in dim light. Only 6 million cones enable us to see colour and fine structural detail. They function in well-lit conditions and become ineffective with diminished illumination. In very low light levels, vision is termed scotopic and light is detected by rod cells of the retina. Rods are maximally sensitive to wavelengths near 500 nm.  In brighter light, such as daylight, vision is termed photopic and light is detected by cone cells which are responsible for color vision. Cones are sensitive to a range of wavelengths, but are most sensitive to wavelengths near 555 nm. Between these regions, mesopic vision comes into play and both rods and cones provide signals to the retinal ganglion cells. The shift in color perception from dim light to daylight gives rise to differences known as the Purkinje effect.

There are three primary colours - red, blue, and green - that make the millions of colours that are distinguishable by the "normal" human eye. Each eye contains three receptors (one for each primary colour) that generate the experience of colour when stimulated in various combinations. This is known as the Young-Helmholtz Trichromatic Theory. Those who have defective cones have difficulty seeing certain colours and are known to be colour-deficient or colour blind.  With this in mind, it is fair to then say that the number of colours the human eye can discriminate depends mainly on the sensitivity of the individual's eyes. The monochromatic colours of the rainbow (red, orange, yellow, green, blue, cyan, and violet) have their wavelengths in the 480-740nm range. When light strikes an object, it can be absorbed, reflected, or scattered and when the surface absorbs all wavelengths equally, we perceive it as black and when the surface reflects all wavelengths equally, we perceive it as white

Visible light is an electromagnetic wave that has a wavelength range of approximately 380 nanometres to 740 nanometres as shown below.


Wavelength Interval

Frequency Interval


~ 625–740 nm

~ 480–405 THz


~ 590–625 nm

~ 510–480 THz


~ 565–590 nm

~ 530–510 THz


~ 500–565 nm

~ 600–530 THz


~ 485–500 nm

~ 620–600 THz


~ 440–485 nm

~ 680–620 THz


~ 380–440 nm

~ 790–680 THz

We often use the HSB, or Hue, Saturation, and Brightness, model to classify colours. The hue of a colour is the particular shade or appearance of a colour. There are 150 hues the eye can distinguish and they include the colours of the visible light spectrum. Brightness refers to the amount of light emitted by an object and saturation is the purity of a colour, or the intensity of a hue. A less saturated colour would be more dull, while a highly saturated colour would be more vivid. The graphic below displays the saturation levels of the colour red, where the bottom has the least saturation.

There is often confusion between the behavior of light mixtures, called additive colour, and the behavior of paint or ink or dye or pigment mixtures, called subtractive colour. This problem arises because the absorption of light by material substances follows different rules from the perception of light by the eye.

A second problem has been the failure to describe the important effects of strong luminance (lightness) contrasts in the appearance of colors reflected from a surface (such as paints or inks) as opposed to colors of light; "colours" such as browns or ochres cannot appear in mixtures of light. Thus, a strong lightness contrast between a mid-valued yellow paint and a surrounding bright white makes the yellow appear to be green or brown, while a strong brightness contrast between a rainbow and the surrounding sky makes the yellow in a rainbow appear to be a fainter yellow, or white.

Colour theory was originally formulated in terms of three "primary" or "primitive" colors—red, yellow and blue (RYB)—because these colours were believed capable of mixing to produce all other colours. This color mixing behavior had long been known to printers, dyers and painters, but these trades preferred pure pigments to primary colour mixtures, because the mixtures were too dull (unsaturated).

For the mixing of colored light, Newton's colour wheel as shown in the course notes is often used to describe complementary colors, which are colors which cancel each other's hue to produce an achromatic (white, gray or black) light mixture. Newton offered as a conjecture that colours exactly opposite one another on the hue circle cancel out each other's hue.

Colour theory has ascribed perceptual and psychological effects to this contrast. Warm colours are said to advance or appear more active in a painting, while cool colours tend to recede; used in interior design or fashion, warm colours are said to arouse or stimulate the viewer, while cool colours calm and relax. Most of these effects can be attributed to the higher saturation and lighter value of warm pigments in contrast to cool pigments.

Gestalt appears again in the study of the importance of colour, its recognition and in how the eye moves around a photograph.  The Gestalt Laws of Organization have guided the study of perception of visual components as organized patterns instead of different parts. As I have blogged in an earlier post, there are six factors that determine how the visual system groups elements into patterns: Proximity, Similarity, Closure, Symmetry, Common Fate (i.e. common motion), and Continuity

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