maintained throughout the year to defend a territory to ensure an adequate food supply, as is the case for reef fishes. This has been shown to be hormonally influenced (Korzan et al., 2008). When males defend territories they are usually bigger and more colourful than their female counterparts. Colour can also sometimes be a warning signal to animals dangerous or of the inedible quality of other organisms, red, black and yellow being commonly employed for this. The black and yellow colouration of bees and wasps represent typical examples. Some animals mimic such warning colours to avoid becoming the prey of others, as examples of selective strategic signalling including “bluffing”, mentioned earlier, clearly show.

Additional Functions

The symbiotic relationship between flowers and butterflies and other pollinators such as bees has evolved so that flowers attract pollinators to feed on their nectar. Unique colourful and/or contrast visual cues, sometimes in the UV range, provide an important ploy used by flowering plants to attract potential pollinators, in the same way that gastronomic restaurants serve meals with visual appeal. The colour of flowers of the horse chestnut tree (Aesculus hippocastanum) can change from yellow to red when nectar is no longer produced, i.e. when there is no longer need for pollination: red is not perceived by pollinating bees although their compound eyes are trichromatic being able to see UV (Wakakuwa et al., 2005; Wakakuwa, Stavenga and Arikawa, 2007). Butterflies tend to avoid the colour green when feeding, but are attracted to it during egg laying, since the next generation of caterpillars requires a good source of food after hatching. The green photoreceptors are thus not used when foraging, but the others (blue and UV sensitive) are critical in reducing the time taken to find nourishment (Spaethe, Tautz and Chittka, 2001). Plants that do not depend on insect or bird pollination are unlikely to have showy or scented flowers. The ultraviolet patches on some butterflies are directionally iridescent, so that they appear to flicker in flight. This flickering is thought to have an important role in butterfly behaviour and communication.

Colour Perception in Man: Context Effects, Culture and Colour Symbolism

It is clear from the intensive research reviewed above that colour plays a major role in the processes which ensure survival, propagation, and every day behavioural patterns of a large number of non-human living organisms. Colour perception is, indeed, the result of a complex evolutionary process that has produced different colour vision systems in various species across the phylogenetic scale, as we have seen above.

How vital is colour perception for man? Is such ability a mere luxury that makes our lives more enjoyable, as some have suggested? Colour signalling is employed everywhere in the world around us, from traffic signals and red warning lights to the bright and colourful advertisements in shops, streets and on the television screen. Research on colour perception in humans tends to suggest that even in man, even though perhaps to a lesser extent than in nonhumans, colour perception plays an undeniable functional role and allows humans to better cope with specific environmental constraints. Also, phenomenological observations and psychophysical data have shown that the way in which we perceive a given colour strongly depends on the immediate visual context the colour is embedded in.

Context Effects in Colour Perception

Colour perception strongly depends on the immediate visual context around the coloured object, its luminance, and its shape. The colour red of a red square, for example, with a constant chromaticity and luminance, is seen as a darker red when against a background with a brighter colour compared with when it is against a background with a darker colour (Fig 10). This well-known observation is called simultaneous colour contrast. Such colour contrast phenomena influence other perceptual processes, such as depth perception and relative impressions of “nearer” and “further away” objects in the visual field (Guibal and Dresp, 2005). The colour red appears to play a particularly important role in such colour based depth perception (see also earlier observations by Bugelski, 1967), and the brain mechanisms potentially underlying this have been linked to both colour stereopsis (Dengler and Nitschke, 1993) and neural interactions in area V4 of the primate visual system (Desimone and Schein, 1987). The functional implications of such a dependency between colour and impressions of near and far may be related to colour based perceptual mechanisms driving attention to detail (Yantis and Jonides, 1991). These may well be equivalent to colour specific perceptual mechanisms in non-human animals, which may help a species detect a prey or predator of a specific colour more rapidly, or may help it assess how far or near it is likely to be.

Figure 10. How a colour is perceived depends on the immediate visual context in which the colour is embedded. The red square on the dark green background shown on top here, for example, is perceived as a brighter than that on the light green background at the bottom. The luminance and chromaticity of all four red squares shown are strictly identical. This phenomenon, also known as subjective simultaneous colour contrast, is accompanied by other visual effects. For example, the red square on the lightest green background here seems nearer to the observer than the red square on the darkest green background.

Colour Perception and Cultural Differences

Colour perception plays an important psychological role in the daily life of human individuals. The importance of colour for humans definitely goes far beyond the “simpler” functions it ensures in non-human animals, such as helping individuals to seek and choose

food, to look attractive to a potential mate, or to deceive or discourage a potential predator. In humans, colour information and perception has assumed a deeper and far more complex psychological significance. It has assumed a symbolic level and as such has become culture dependent in the sense that what “blue” may evoke in a European is not necessarily what it may evoke in a Chinese or a member of a remote African tribe. Research on colour cognition across cultures has generated substantial evidence for differences in colour cognition between language communities, including semi-nomadic African tribes, revealing a considerable diversity in the way different languages partition the physical continuum of colours visible to man. While some languages have been reported to use as few as two terms to describe all visible colours, others use up to twelve. Whatever the origin of these differences, it appears that even when two different languages have the same number of terms for similar points in colour space, there are significant differences in their cognitive organization as a function of culture and visual environments (Roberson et al., 2005). Cognitive organization or mapping of perceived events to metaphors, or terms of a given language may lead human individuals to contradict input from visual perception when judging the brightness of physical objects, for example (Meier et al., 2007). Emotions in particular seem to influence how the colours we perceive may be given a cognitive interpretation that is detached from visual perception as such. Thus, when making category judgements (Meier, Robinson, and Clore, 2004), people seem to have a tendency to assume, possibly unconsciously, that objects with bright colours are “good”, “uplifting”, and “forward”, whereas objects with darker colours are “bad”, “depressing”, and “obscure”.

Colour Symbolism and Emotions

In the most general terms, we tend to call colours “beautiful” or “ugly “, “warm” or “cold”, “gay” or “sad”, but we also go well beyond such categories. Colours may indeed evoke a variety of emotions, ranging from surprisingly strong to subtle and rather finely tuned. Red, the colour of blood often provokes an intense reaction, symbolising the passion and destruction indicated by it. It also often represents the forbidden, danger, even social revolt and, thus, courage. Pink, in contrast, symbolises noble emotions such as love, affection tenderness, femininity, babies and, more generally, harmony. Violet is associated in most cultures with spiritual peace, meditation and religious reflection and as such confers the notion of mystery. Purple, close to violet, adds a “noble” aspect to the latter and as such becomes the colour of kings and noblemen and social and spiritual power. Blue is the colour of the sky and stretches of water and thus suggests ideas of space and serenity. At the same time it provokes a sensation of cold. Green is found everywhere in Nature and symbolises life, freshness, vitality, health and optimism. It does not strongly stimulate the emotions and thus is a calming colour. Yellow is a luminous stimulating colour representing joy, but can provoke many different emotions. Orange, a colour close to that of flames induces a feeling of warmth (either physical or human). Brown, an essential colour of soil evokes honesty and as such feelings of comfort and stability. Black is a colour closely associated with night and therefore with death, bad luck, despair and sadness. Nevertheless in modern European society, black can also represent modernity, elegance and “chic”. Grey is a totally neutral colour symbolising equilibrium and confidence. White at the opposite extreme to black and devoid of sensuality is the icon of psychic and physical purity, of innocence and light.