IT'S ALL IN YOUR HEAD:

Colour is first and foremost an experience: it exists only so far as we are here to look at it, and nobody looks at colour in exactly the same way. The human eye possesses around 100m photoreceptors responsible for absorbing photons, the particles of energy that make up light. As these photons enter the eye, they trigger a rapid chain of events in the brain that ends in our ability to see, and make visual sense of, the world.

Only about four or five million photoreceptors, called cone cells, are responsible for colour vision. Most people have three types of cone cell, each of which picks up on a different wavelength of visible light. By comparing these different wavelengths, the brain can distinguish between millions of different hues and shades of colour. But everyone perceives colour slightly differently. Some people have fewer cone cell types than others, and therefore perceive fewer colours; others have a fourth type of cone cell, and so possess an "extra dimension" in how they see colour.

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