What’s the colour of magic?

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There are some colours that exist but we cannot see. But researchers are getting closer to understanding how we might glimpse them, writes Mark Eltringham.

In the Discworld novels, Terry Pratchett introduces us to the colour of magic. He calls it octarine, a sort of greenish purple, described as ‘the undisputed pigment of the imagination’. Surprisingly such unseeable colours exist for the human eye. They are invisible to us because of the limitations of our vision. Some invisible colours exist outside the perceptions of the human eye, including large parts of the infrared and ultraviolet spectrum. We cannot see them because, compared to many other animals, the human eye is only capable of perceiving light across a narrow band of wavelengths.

Yet there are some within this band that we are still unable to perceive. These are the forbidden colours, which we tend to describe only in relation to others. So we might describe them as reddish-green or blue-ish yellow, but we should be careful we don’t assume they would then be like the results of a mix of those colours as pigment or paint. They exhibit characteristics of both frequencies which makes them fascinating as ideas but imperceptible to the eye.

The reason we cannot perceive some colours is down to the way the cone receptors in our retinas limit our vision. There are three types of cone cells connected to nerve impulses through opponent neurons which respond to different colour wavelengths while inhibiting the response to others. So the same cone cells that detect red light, dampen our response to green. The cones that detect yellow, inhibit blue. And a third set of neurons distinguish between black and white.

While the vast majority of colours induce a mixture of effects in these sets of neurons, which our brains decode, pure red light exactly cancels the effect of green light and yellow exactly cancels blue, so our brains should never be able to perceive those colours as coming from the same source.

“Some invisible colours exist outside the perceptions of the human eye, including large parts of the infrared and ultraviolet spectrum.”

However some scientists claim we can discern such impossible colours if we learn how to look. In 1983, two researchers called Hewitt Crane and Thomas Piantanida published a paper in the journal Science called ‘On Seeing Reddish Green and Yellowish Blue,’ which claimed for the first time that these forbidden colours could be perceived by the eye.

The researchers created pictograms in which red and green stripes or blue and yellow stripes ran next to each other. They showed the images to volunteers, using an eye tracker to hold the images fixed to ensure that light from each colour was picked up by the same parts of the retina.

What they found was that the subjects of these experiments reported that the borders between the stripes in the images gradually disappeared and the colours themselves started to merge to form hues they had never seen before. The researchers concluded that by staring at the images, the brains of the subjects had learned to overcome the opponent mechanism in their eyes.

The experiment was updated in 2006 by scientists at Dartmouth College in the US using computer images and concluded that what people are seeing in the experiment are not forbidden colours but a mix. The experiments remain controversial with researchers who are still learning about the eye as well as the characteristics of colour and light. But maybe one day they will reach a consensus not only about the exact nature of the forbidden colours, but also a way for us to see them easily and so open our eyes to colours about which we can still only dream.