The world the brain presents in spectacular colours is in fact colourless. The term colour is only meaningful in regard to a living organism with sensory cells for light. These sensory cells have developed differently for each species – not all species see one and the same object in the same colour. Nonetheless – colour is one of the most important perceptions and is associated with the following: emotion, information and communication.
A world in colour is the product of our brain. Colour is not a physical property of an object - it is a sensation, just like smell or taste. Colour is generated only when light of a particular wavelength falls onto the retina of the eye and specialized sensory cells generate a nerve impulse, which is routed to the brain where it is perceived as being colour.
Humans are trichromats, which means they possess three different types of colour receptors – for red, green and blue light – and are able to distinguish a wide range of colours, approximately 10 million in total.
All colours that humans can see are composed of red, green and blue. For a mammal, humans see exceptionally well, given that most mammals are not equipped with such good colour vision. They have partially unlearned colour perception. Many other species, such as reptiles and birds outperform humans, many of which are tetrachromats.
Colour vision is differently well developed in various species, which is a direct result of the fact that in the history of evolution vision has developed independently – adapting to varying living conditions. Monochromats such as whales or seals rely only on one receptor, while most mammals have two receptors (dichromats), humans and primates posses three receptors (trichromats) and birds and some reptiles and many fish have even four receptors (tetrachromats). Most species’ visual organs are sensitive to the wavelength of light that is visible to humans, while others, in contrast to humans, can also perceive ultraviolet or infrared wavelengths.
Birds see the world in more colours than we do. They are also able to perceive ultraviolet light. This is particularly important when it comes to mating. The plumage of many bird species has reflecting UV patterns that remain invisible to the human eye.
Bees see the world with different eyes too. They also perceive light of the ultraviolet spectrum. While some flowers may seem rather inconspicuous to us they bear clear markings for bees. These UV patterns are for pollen and nectar gatherers clear signals and facilitate their orientation. Some flowers even mark ideal landing spots.
Given that there are also genes for red and green receptors on the X chromosome, tetrachromacy has been speculated in humans too. Since women have two X chromosomes, through mutation they could develop an additional colour receptor. Would however a fourth colour receptor in females induce changes in the visual centre so that the expanded spectrum could be translated into colour sensations and made visible? Overall, only a few cases of humans with four colour receptors have been suspected; none of which have been however confirmed. Evolution has not yet come to end and who knows what we will get to see in the future….