Guardian, Thursday 15 March 2007

Like those jingles you can’t stop humming, some bad ideas stick. This one has maddened me for years: when you and I see a green ball, do we see the same green? When we have toothache, we don’t all have the same toothache. The notion that pain varies between individuals does not disturb us. Why, then, do we resist the idea that different people see different colours?

Just as you and I, each suffering our own very different toothaches, can agree on what a lousy experience toothache is, so we can all roughly agree on what colour is what. We can argue till the cows come home whether this particular shade of turquoise is green or blue, but we both pretty much agree on what green and blue are. There is a lawfulness to colour, and it would help if we knew where this lawfulness resided.

In his 1995 essay The Case of the Colour-blind Painter, the neurologist Oliver Sacks describes the case of an artist who, through subtle but devastating damage to his brain, could not see colour. Though damage of this sort robs an individual of the experience of colour, the mechanisms of colour vision continue to function. Asked to match up coloured counters, people with no experience of colour are still able to match up colours perfectly. They just don’t see them. But if the relationship between wavelength and colour is purely contingent, where the devil do colours come from?

Artists are forever trying to uncover universal meanings behind their colours. It is easy to scorn their efforts, not least because this kind of thinking dates very quickly. Kandinsky’s experiments in colour symbolism may as well have been conducted in the 14th century for all their relevance now. There is, none the less, a growing body of evidence that colours, shapes, sounds and smells do have meanings. Wolfgang Köhler’s delightfully simple 1929 experiment asked volunteers to match a pair of abstract figures to one of two nonsense words, “maluma” and “takete”. Immediately, and virtually without exception, people matched maluma to the soft round figure and takete to the sharply angular one. Some sort of shared symbolism related the sounds to the shapes.

Now Dr Jamie Ward, at University College London, might have uncovered an underlying symbolism to colour. Ward’s interest is synaesthesia – the experience of a handful of individuals who perceive information through an unexpected sense. Some hear colours, others smell shapes. The vast majority see sounds. The experiences of individual synaesthetes are notoriously idiosyncratic. But there are unexpected regularities, and Ward’s bulging address book – he knows 450 synaesthetes by name – allows him to spot trends that were formerly invisible. For example, among synaesthetes who see coloured letters, A is often red, B is often blue, and C is often yellow. “This is likely to hold true for other types of synaesthesia,” Ward says, “assuming that we are able to make a large enough number of observations. For instance, certain musical instruments may tend to produce particular colours, shapes and movements.”

Synaesthesia may simply be an exotic manifestation of something we all enjoy: the ability to turn sensations into symbols, and to think with them. After all, if our thoughts are not made of sensations, what are they made of? And this is why we find it so distressing, you and I, to realise that we don’t see the same colours. Colours – so striking, so beautiful, so manifestly there – are one of the few things we can agree on, more or less. How cast adrift will we feel if colours turn out to be, after all, only our thoughts about light?