There’s a neurological model of colour perception that leads to surprising conclusions about colours, and interesting parallels with emotions.
The 2005 paper Chimerical Colors (PDF) describes a standard model of how humans perceive colours, the Hurvich–Jameson opponent-process network. The model is convincing. It provides a good explanation of colour perception, explaining the observed fasts about how people see colours.
But the model goes further, predicting that we ought to have the ability to see colours that do not exist in nature. And the paper goes even further, including actual examples of experiments you can perform on yourself to see colours that don’t exist.
The general principle is that when you gaze at a strong colour for a while, it fatigues the part of your brain that responds to that colour. When you transfer your gaze somewhere else, you see a negative after-image of the colour you were gazing at.
For example, gaze steadily at the white + sign in the middle of this blue square, close up for about 20 seconds or until you start to see ghosting around the edges. Then transfer your gaze to the + sign in the pale blue square. You see a white after-image where your ability to perceive blue was fatigued. The fatigue gradually wears off.
There’s nothing chimeric about white, but when you do the same with this pair the red fatigue you create makes you see a blue-green after-image on the black background. In the real world black cannot have a hue. There is almost no blue or green light reaching your eyes from the black area. So this blue-green yet black colour is chimeric — impossible in nature but visible to humans.
The paper is technical but it explains the model very clearly. For more examples of chimeric colours, scroll down to the diagrams starting with Figure 11 on page 22 of the PDF.
The way these after-images work is broadly similar to the CBT model of emotional disorders.
Some circumstance produces a cognitive bias in your perception of the world, so that your emotional response to the world no longer represents the way the world really is. Your emotional response might even represent a world that’s impossible.
Normally, the cognitive bias would be temporary, and as it wears off your emotions return to normal. That is, they return to representing the world accurately. But in mental illness the cognitive bias gets stuck. You continue to have a biased emotional response. This is like continuing to see the blue-green after-image for months after you gazed at the red square.
The discovery that made CBT possible was that the emotions are not a direct perception of the world in the same way that colour is. Emotions are an indirect perception mediated by thoughts.
So CBT works by identifying the particular bias that you have somehow got stuck with, and then using thoughts to undo the bias. The end result is that you can once again experience a full range of emotions without bias.
For example, we might say that suffering from depression is like seeing a blue after-image everywhere. The aim of treatment is not that you should never see the colour blue again, making your whole world yellow. The aim is that you should only see the colour blue when the thing you are looking at really is blue.
Or in other words, after treatment you should be able to become as depressed as the next person when something depressing happens to you. CBT only removes the after-image of that past circumstance, so that it no longer colours everything.