I would like to discuss the issue of whether colour exists or not, from a philosophical perspective. Speaking more strictly I am going to be writing about the nature or ontology of colour, since to argue that it doesn’t exist at all would be somewhat damning on my career as a colour scientist to date.

Before you continue I suggest you make yourself a strong black coffee, dim the lights, and relax to avoid the thumping headache that could result from reading further without taking these precautions.

In a previous blog (http://colourware.wordpress.com/2009/06/29/colour-101/) I wrote about the putative relationship of wavelengths of light with colour. One view of colour ontology is objectivism; that is, that objects are coloured and that the colours of objects can be identified with the composition (wavelengths) of light reflected or with the reflectance factors of objects. Certain wavelengths can be associated with certain colours and objects have certain colours because they reflect certain wavelengths of light and absorb others. Simply put a red object is red because it absorbs the short wavelengths of light and reflects (or transmits) the longer wavelengths. However, it is easy to show that the colour of an object (for example, a patch in a scene) is not invariant; rather, it changes with the surrounding or background colours (as shown below).

In this example, the two central squares are physically identical in their spectral properties but appear to be different colours. Metamerism would also seem to be troublesome for objectivism. Metamerism commonly occurs, for example, when two objects reflect different wavelengths compositions but are indistinguishable in colour when viewed under a particular light source; crucially, when seen under some other light source the two objects no longer match each other in colour. We also know that the same object will look different in colour to a so-called colour blind observer (~10% of the male population are colour blind) compared with a so-called normal observer. Objectivists could counter this by saying that objects are coloured and one can equate colour with physical properties – it is just that we need to define standard conditions. But there is great difficulty in defining what those standard conditions are.

Thus, although some philosophers still argue for colour objectivism, many reject it – including, for example, Evan Thompson who wrote a fantastically entertaining and informative book on this very subject (http://www.amazon.co.uk/gp/search?index=books&linkCode=qs&keywords=0415117968). It would seem that when Newton famously wrote that ‘the rays are not coloured’, he was also rejecting objectivism though there is some lack of clarity from Newton’s writings on this matter.

The natural opposite of objectivism is subjectivism; this takes the view that things are coloured only in so far as they have the disposition to cause sensations of colour in a perceiver.

An extreme form of subjectivism is called extremism; according to this view nothing is strictly speaking coloured at all, not even dispositionally. Colours are entirely in the head; they are nothing but sensations of a certain type. This is the view that I adhere to. Colour is a sensation that results from a biological process that occurs in our brains and, presumably, in the brains of many other species. I was once challenged by a famous American lawyer on this point; it’s a long story why this happened, but suffice to say he asked me whether I believed that if a tree fell in a forest and there was nobody there would it make a sound? Although at the time I managed to side-step this difficult question I can state here that I do not think it would make a sound. I believe that when an object ‘makes a sound’ it causes wave-like vibrations in the air and that our auditory systems detect these vibrations, convert them into neural signals, and ultimately result in a neural state that results in the listener experiencing a sound. Without a listener there can be no sound. Similarly, objects reflect wavelengths of light, these wavelengths are detected by our visual systems … and we experience colour. To me, a planet without life would have no colour, no sound, no taste etc and arguing otherwise is like arguing that that planet would have pain or fear.

However, it is not straight forward that if we reject objectivism we should embrace subjectivism. There are arguments that can be used to reject the extreme form of subjectivism that I believe in. Although at first it may seem obvious that colours are either properties of objects or ‘in our heads’ Thompson suggests that colours could be relational properties, not intrinsically linked to any item. According to this view there would be no perceiver-independent account of colour but neither would colours be reduced to mental or neural states. Rather, colour would be a relational property, resulting from the relationship between objects and observers.

Ultimately I believe it is possible to make a case for any of these views about the ontology of colour. The truth may well be somewhere between objectivism and subjective extremism. So why should I be so passionate about arguing for subjective extremism? The answer is that over 20 years of teaching students about colour has led me to the view that the notion that colour is a fixed and invariant property of objects is a barrier to their learning. This notion that they have originates, and is constantly reinforced, by our use of language when we say, for example, that “this book is red” or “that pencil is yellow”. Whenever they come across a situation when an object seems to change colour (as I have shown, this happens when we change the colour of surround and in many different situations) they dismiss it as an illusion. This prevents them from easily understanding some important concepts in colour education.  

Thus I would say that when an objects changes colour because of the background or the light source, it’s not an illusion. Rather, it’s an illusion to think that objects have a fixed colour.