What shapes the way we experience colour?
Universal features of the world around us such as blue skies, red fire, and green vegetation are responsible for colour associations which transcend cultural boundaries. Research consistently reports blue to be the world’s favourite colour, for its connotations of calm and cleanliness. Meanwhile, red evokes a physical reaction of alertness, and is thus globally used in stop signs and on emergency vehicles.
(Holi Festival of Colours at CERES.)
Cultural factors are also responsible for moulding colour associations. The history of purple is a fascinating example of this. Purple fruits, flowers and animals are rare in nature, so few prehistoric humans would ever encounter the colour. The earliest purple dyes required 9000 shellfish to be crushed per gram of pure pigment, so purple fabric was literally worth it’s weight in gold! From this arose a strong association between purple and luxury. Sumptuary Laws dictate which classes of people can wear certain colours and apparel, and in both Roman and Elizabethan culture the wearing of purple garments was restricted to royal and imperial social classes, heightening the colour’s glamour and exclusivity.
In 1856, the chance mishap of an 18 year old chemist resulted in production of the first synthetic purple compound, mauveine. Elite purple became available for mass consumption, and was an absolute sensation in the Victorian era, even being used to dye ‘penny lilac’ stamps between 1881-1901. However, as the circumstances of purple have changed, so have the associations. Luxurious purple has become a marketing cliche, which is reflected in contemporary branding research. US respondents now associate purple packaging with inexpensive products.
What is colour in the first place?
Our most accurate definition states, “colour is the visual effect that is caused by the spectral composition of the light emitted, transmitted, or reflected by objects”. Light itself can be characterised by wavelength, where shorter wavelengths correspond to higher energy beams of light radiation. The human-visible wavelength range is 400-700nm, and the colours corresponding to each wavelength are shown in the image below.
(Showing the range of wavelengths each of the human photoreceptors can detect.)
Light emitted from a source such as the sun contains many different wavelengths. When this light hits an object, the composition and pigmentation of that object determine which wavelengths of light it absorbs. The remaining wavelengths of light are reflected back, and the composition of this reflected light determines an object’s perceived colour. For example, red objects reflect red light. The combination of all visual wavelengths produces white light, as shown in the image below.
(Illustration of how the reflected wavelengths of light are responsible for the observed colour of objects.)
The mechanism by which eyes detect and interpret the light they receive is extremely intricate. Human beings are trichromats, meaning we possess three types of colour-sensing receptor in our eyes. Each type of colour receptor contains a pigment molecule which absorbs a different range of light wavelengths. S (short wavelength) receptors absorb mostly blue light, M (medium wavelength) receptors absorb mostly green light, and L (long wavelength) receptors absorb mostly red light. When a beam of light hits the back of the eye, the relative absorbance by each type of receptor is assessed. A beam of pure red light would be detected by strong activation of L, weak activation of M, and no activation of S.
Do other species perceive a different range of colours?
Almost certainly. Bees are trichromats, like humans, but their visual range is shifted down to between 300-690nm. The lower limit of their vision (300nm) is down into the UV range, allowing bees to identify flowers for pollination by UV pigmentation on their petals.
One of the most extraordinary visual systems is that of the mantis shrimp. It possess 16 unique types of colour-receptive cones, and a has multiple compound eyes. A human being has two eyes, each independently focusing, giving us binocular vision. However, each eye of the mantis shrimp is made up of three distinct regions, giving the organism trinocular vision! This means that the mantis shrimp can perceive depth using a single eye. Each eye is mounted on an independently movable stalk, and their vision is extremely sensitive, extending into the UV range. Evolutionary advantages of this system include better hunting (mantis prey are often semi-transparent), and the ability to identify the phase of the moon from within the shallow pools they inhabit (mantis fertility varies with the lunar cycle).
Could humans ever expand their visual range?
Research into mammalian vision suggests that not only is this possible, but it could be as simple as introducing a single gene! Most mammals (excluding primates) are dichromats, possessing only S and M photoreceptors. They possess vision similar to that of red-green colour blind humans. Scientists genetically modified a group of dichromat mice so that they could produce the human L photoreceptor. This would theoretically give them trichromatic vision, allowing them to differentiate between red and green.
Experiments were carried out to determine whether mutant mice had expanded colour vision. Mice were placed in an enclosure containing three lit-up panels, where two panels displayed the same colour, and the other was different. There was a tap above each panel, but only the one above the different panel dispensed a drop of soy milk, rewarding the mice. Researchers anticipated that despite possessing biologically active photoreceptors, the mice might not be able to interpret information from them, so could remain functionally dichromats. However, while normal mice only chose the correct panel 1/3 of the time (consistent with random guessing), mutant mice identified the different panel 80% of the time. This suggests that mammals possess the neural machinery allowing them to interpret input from entirely new light receptors.
The ethical considerations of modifying humans to expand our colour perception are controversial, but this exciting possibility is further within our reach than ever before. It is safe to say that our future is looking colourful!