Are There Some Colours You Can’t See?

Some researchers think there are colours that we humans are unable to see. They are also referred to as "impossible colours”. However, six people may have seen them during a scientific experiment.

Our eyes have only three types of colour receptors, or "cones". There is one type that is sensitive to red light, another picks up blue light and the third detects green light. The colour "purple" is therefore not perceived as a single colour, but it activates both the blue and red cones, and our brains then generate a mixture of the two.

Impossible colours

This method works quite well, but there are still some colours that our brain cannot easily combine. These are known as "impossible colours”. One example is "blue-yellow”. We don't mean "green", which is a mixture of blue and yellow. We mean a colour that really is both blue and yellow.

Blue or yellow

So why is “blue-yellow” – and also a colour like “red-green” – impossible for us to perceive? It is all about receptors and the human brain. Cones transmit information from their blue, green and red receptors to nerve cells, which in turn transmit the colour signals to the brain.

At that moment something remarkable happens: the nerve cells suddenly turn them into four colours. These are not only blue, green and red, but yellow as well. According to the "colour opponent process theory", only two nerve pathways are used to transmit those four colours to the brain: one for blue and yellow and one for red and green. Each nerve pathway can only transmit one colour at a time, so it can transmit either blue or yellow at one time, not "blue-yellow".

Six “seers”

Scientists are always up for a challenge, so researchers working for the U.S. Air Force tried to trick the brain into perceiving impossible colours. We don't know many details because the tests are classified as US military secrets, but the scientists did reveal that six of the seven subjects successfully saw the colours "blue-yellow" and "red-green”. They were able to clearly remember this colour for several hours after the experiment, although they did find it difficult to describe.

Truly impossible?

Some researchers, however, claim that these impossible colours are easy to see. They say that red-green, for example, is simply a mixture of red and green. In a 2006 study by Po-Jang Hsieh and his team at Dartmouth College, the subjects identified brown (the mixed colour) as red-green.

Impossible Colour Test

Do you want to try and spot an "impossible colour" like blue-yellow at home? You need an area of blue and an area of yellow colour that are both equally bright.

Put your nose on the dividing line between the coloured areas and keep staring at the line where blue meets yellow. With a bit of luck you will begin to see a bright new colour that is difficult to describe: this is “blue-yellow”.