The world is full of colour, from the deep blue of the ocean to the vibrant red of a rose. But what we perceive as colour is not an intrinsic property of objects; it is the result of how our eyes and brain interpret different wavelengths of light. Light travels as electromagnetic waves, and the visible spectrum ranges from about 380 to 750 nanometres. Different wavelengths correspond to different colours: short wavelengths appear violet, medium wavelengths green, and long wavelengths red. When light reflects off an object, it enters our eyes and triggers a complex biological process that ultimately allows us to see colour.
This process involves several structures within the eye, each playing a crucial role in transforming light into the colourful images we experience every day. The eye functions much like a camera. Light first passes through the cornea, a transparent layer that helps focus the incoming light. It then travels through the pupil, the dark opening in the centre of the iris, which adjusts its size to control the amount of light entering. Behind the pupil, the lens further focuses the light onto the retina at the back of the eye.
The retina contains millions of specialised cells called photoreceptors. There are two main types: rods, which are highly sensitive to low light levels and detect shades of grey, and cones, which are responsible for colour vision. Cones are concentrated in the fovea, a small central area of the retina, and are less sensitive to dim light, which is why colours appear muted at night. Human colour vision relies on three types of cone cells, each sensitive to a different range of wavelengths. These are often called S-cones (short wavelength, sensitive to blue light), M-cones (medium wavelength, sensitive to green light), and L-cones (long wavelength, sensitive to red light).
This process involves several structures within the eye, each playing a crucial role in transforming light into the colourful images we experience every day.
This is the basis of the trichromatic theory of colour vision, first proposed by Thomas Young and refined by Hermann von Helmholtz. When light enters the eye, each cone type responds to a certain degree depending on the wavelength composition of the light. The brain then compares the signals from the three cone types and interprets the combination as a specific colour. For example, yellow light stimulates both M- and L-cones, so we perceive yellow. This additive process allows us to see millions of distinct colours from just three types of receptors.
However, trichromatic theory alone cannot explain all aspects of colour perception, such as afterimages. For instance, if you stare at a red patch for a long time and then look at a white wall, you see a green afterimage. This phenomenon is better explained by the opponent-process theory, proposed by Ewald Hering. According to this theory, colour perception operates through three opponent channels: red-versus-green, blue-versus-yellow, and black-versus-white (for brightness). Each channel responds in an opposing manner; for example, when the red-green channel is excited by red, it is inhibited from responding to green.
After prolonged stimulation by red, the channel becomes fatigued, and when you look at neutral white, the green opponent side becomes more active, producing a green afterimage. Both theories are now understood to work together: trichromatic processing occurs at the level of the cones, while opponent processing happens in the retinal ganglion cells and the brain. Colour blindness affects approximately 8% of men and 0. 5% of women of European descent. The most common form is red-green colour blindness, which results from a deficiency in either M-cones or L-cones. People with protanopia lack functional L-cones, making it difficult to distinguish reds from greens because they perceive these colours as similar shades.
Deuteranopia is a lack of M-cones, causing a similar confusion. A much rarer form, tritanopia, involves missing S-cones and affects blue-yellow discrimination. In most cases, colour blindness is inherited through a recessive gene on the X chromosome, which explains its higher prevalence in males. While colour blindness can pose challenges in everyday life—such as reading traffic lights or interpreting colour-coded information—many affected individuals adapt and use brightness or position cues to compensate. Interestingly, the way we talk about colour varies across cultures and can influence how we perceive it.
The classic studies by Berlin and Kay in the 1960s identified a universal pattern: all languages have words for black and white, and if a language has a third colour term, it is almost always red. Additional terms emerge in a predictable order—green and yellow, then blue, and so on. However, the precise boundaries of colour terms differ. For example, ancient Greek lacked a distinct word for blue, and some languages combine blue and green into one category (like the Welsh word 'glas'). Some researchers argue that language can shape perception, as suggested by the Sapir-Whorf hypothesis.
Studies show that speakers of languages with more colour terms can discriminate between subtle colour differences more quickly, though the effect is debated. Understanding colour perception has practical applications in many fields. In design, knowledge of colour theory helps create visually appealing and accessible graphics, ensuring that colour blindness is considered in user interface design. Traffic lights universally use red, yellow, and green because these colours are most easily distinguished, even by some colour-blind individuals when combined with position. In art, painters like the Impressionists exploited the science of colour by using complementary colours to create vibrant contrasts.
Scientists and engineers use colour perception to develop displays, printers, and cameras that accurately reproduce colours. Even in medicine, colour tests can help diagnose vision problems or detect early signs of eye disease. Our ability to perceive colour is a remarkable biological gift that continues to fascinate researchers and enrich human experience.