The news that there are “colours” that you cannot see should not be new news to you. The idea that mixtures of colours based off the visible spectrum that you still cannot see may be however. Certain colours, such as pink, are mixtures of different wavelengths of light, but other colours that are mixtures simply cannot be perceived and sound a bit like a real life octarine. These are colours such as red-green and blue-yellow, which are not actually what you get when you mix the two, but really a reddish variety of green or a bluish yellow colour. So why can’t we see these colours? The answer lies in what are known as “opponent neurons” in the eye’s retina. When red is seen one type of these neurons will fire, which the brain sees as red, when green is seen the neuron is silent and this lack of signaling is perceived as green. Interestingly this is also the basis of Red-Green colour blindness. So although these colours actually exist, we cannot see them because we cannot have a neuron firing and not firing at the same time.