The human eye is a remarkable organ that allows us to perceive the world in vivid colour. But how exactly does it turn light into the rich palette we experience? The process begins when light enters the eye through the cornea, a transparent dome that helps focus the light. It then passes 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, a light-sensitive layer at the back of the eye.

The retina contains millions of specialised cells called photoreceptors, which convert light into electrical signals that travel to the brain via the optic nerve. The brain then interprets these signals as colour and shape. There are two main types of photoreceptors in the retina: rods and cones. Rods are highly sensitive to light and allow us to see in dim conditions, but they do not detect colour. Cones, on the other hand, are responsible for colour vision and function best in bright light. Most humans have three types of cones, each sensitive to different wavelengths of light: short (blue), medium (green), and long (red).

This is known as trichromatic vision. When light hits the retina, each cone type responds to its specific range of wavelengths, and the brain combines these signals to create the full spectrum of colours we perceive. For example, yellow light stimulates both red and green cones, and the brain interprets that combination as yellow. The trichromatic theory explains how we see colour, but it does not account for all aspects of colour perception. Another important concept is opponent-process theory, which suggests that colour vision is controlled by three opposing pairs: red versus green, blue versus yellow, and black versus white.

The retina contains millions of specialised cells called photoreceptors, which convert light into electrical signals that travel to the brain via the optic nerve.

According to this theory, cells in the retina and brain process colour by comparing the signals from different cone types. For instance, a cell might be excited by red light and inhibited by green light. This explains why we cannot see a reddish-green colour—the two colours are opposite and cancel each other out. Opponent-process theory also explains afterimages: if you stare at a red object for a long time and then look at a white wall, you see a green afterimage because the red-sensitive cells become fatigued. Colour vision deficiencies, commonly called colour blindness, occur when one or more types of cones are missing or malfunctioning.

The most common form is red-green colour blindness, which affects about 8% of males and 0. 5% of females of Northern European descent. People with this condition have difficulty distinguishing between red and green hues because their red or green cones are not working properly. Less common are blue-yellow colour blindness and complete colour blindness, where a person sees only shades of grey. Colour blindness is usually genetic, caused by a recessive gene on the X chromosome. This is why it is more common in males—they have only one X chromosome, so a single faulty gene causes the condition, while females need two faulty copies.

Beyond the basic biology, colour perception is also influenced by the brain's interpretation. The same physical colour can look different depending on the surrounding colours, a phenomenon known as colour constancy. For example, a white piece of paper appears white whether it is in sunlight or under a lamp, even though the light reflected from it is actually different. The brain automatically adjusts for the lighting conditions to maintain a consistent perception. Another example is the famous dress illusion, where some people saw a blue and black dress while others saw white and gold.

This happens because the brain makes assumptions about the lighting—whether the dress was in shadow or bright light—and adjusts the colours accordingly. The evolution of colour vision in humans is closely tied to our ancestors' diet. Primates, including humans, developed trichromatic vision to help identify ripe fruits against a background of green leaves. Many fruits are red or orange when ripe, and being able to distinguish these colours from green foliage provided a survival advantage. In contrast, most mammals have dichromatic vision, with only two types of cones, which is why dogs and cats see fewer colours than humans.

Birds and insects often have tetrachromatic vision, with four types of cones, allowing them to see ultraviolet light. This helps them find nectar in flowers that have UV patterns invisible to humans. Understanding how the eye sees colour has practical applications in many fields. In medicine, colour vision tests are used to diagnose eye diseases and monitor treatment. In design and art, knowledge of colour theory helps create visually appealing and accessible content. For example, designers must consider colour blindness when choosing colour schemes for websites and signs to ensure information is clear to everyone.

In technology, cameras and screens are designed to mimic human colour perception, using red, green, and blue pixels to create millions of colours. As research continues, scientists are exploring how to restore colour vision to people with deficiencies, using gene therapy or special contact lenses. The human eye's ability to see colour is a complex and fascinating process that combines physics, biology, and psychology.