To assess the rival claims about the status of colour, we need to clarify what the aim of a philosophical theory of colour should be. Philosophical discussions of colour are sometimes framed in terms of answering such questions as "what is the nature of colour?" "what is colour essentially?" "what is the essence of colour?". Sometimes, they are framed in terms of answering the question of what kind of understanding a person must have in order to understand colour concepts or to be able to use colour terms with understanding. Here we are asking a question about colour concepts and it will be important to clarify first, whether the concepts in question are concepts are concepts of natural language, or technical concepts introduced for scientific or industrial purposes, and then with respect to both, whether there are different kinds of concepts.

On the face of it, there are two different exercises here: identifying the nature of colours, i.e., what colours are essentially; and specifying what the concept of colour is. It seems that one exercise requires looking at the world and the other looking at the thinkers. However both exercises would seem to be an integral part of any philosophical theory of colour. For there appear to be two prominent facts about colours that any theory would need to respect: (1) that colours are properties in the world (i.e., properties of physical objects), to which one's colour vision is sensitive; (2) that colours are qualities that perceptual experience represent (or presents) objects as having. At least, if any theory denies that these are facts about colours, then an extremely good explanation is called for. One theory that comes close to denying that the first is a fact is the Descartes-Locke tradition. This theory is more subtle, however, than this would suggest. It draws a distinction between two concepts or senses of colour. In one sense objects do have colours but this is not the sense in which objects are represented as having colours; while in the other sense, objects are represented as having colours, but these are not properties which objects actually have.

There is a stronger reason for thinking that the two exercises might be related. This reason depends on the fact that there are different available models for thinking about concepts. For certain kinds of concepts, the understanding required in order to possess the concept of X provides the only answer to the question of what X's are essentially. (The nominal essence is the same as the real essence.) On the other hand, if externalists are right then the content of some mental states are broad. Hence, if concepts are constituents of the content then individuating these concepts will require identifying some object, property or natural kind. Accordingly, depending on which model for colour concepts we adopt, the two exercises (looking at the world and looking at the thinkers) may be related.

In any case, if those in the Descartes-Locke tradition are right in drawing distinctions between different concepts of colour, then there will be different answers to the question of what the essence of colour is, depending on which concept we have in mind. There are, within this tradition, two important distinctions. One is between colour as a property of physical bodies and colour as a quality in experience, i.e a phenomenal or subjective quality: colour-as-it-is-in-experience. In the second place, if we concentrate on colour conceived as a property of physical bodies, we can distinguish between a naive, pre-reflective concept and a sophisticated, critical concept. For Descartes and Locke, the former concept is confused or at least has a faulty assumption built into it. Accordingly, the answer to the question of what colours are essentially, will be "none" for the pre-reflective concept and "a mind-dependent dispositional property" for the critical concept.

Much of the opposition to the Descartes-Locke account consists in challenging their characterisation of the natural or folk concept. Some philosophers, Dummett, Evans, McDowell, argue that the natural concept is dispositional and does not need to be reconstructed in the way that Descartes and Locke suggest. A different form of opposition is found among the many objectivists who claim that the natural concept is objectivist. Opinion divides between those who hold that colours have hidden essences, e.g., microstructural features such as spectral reflectances, and those who hold that their essences are manifest. Most modern objectivists believe not only that colours have essences but that these essences have either been discovered by scientists, or are close to being discovered.

It is possible, however, to take a different view: that the natural concept of colour is objectivist, that is, that colours are conceptualised as objective, intrinsic features of physical bodies but, so it happens, there are no such features in nature. The essences are virtual, not actual. On this account, colours are virtual properties (have virtual essences) just as phlogiston and caloric are virtual natural kinds. Given that colour perceivers have experiences of colour, i.e., experiences which represent objects as having colours, this account implies an `illusion theory' or `error theory' of colour experience and perception. Objects are perceived as having colours which they do not in fact have: there are no such colours.

However the dispute about the natural pre-reflective concept is resolved, we still need to answer the question of how we ought to think of colour. If Descartes and Locke are right, then the natural concept needs to be replaced by the critical concept. However, even if they are wrong and dispositionalists such as Dummett and Evans were right about the natural concept, we might want to argue that the concept should be either replaced by, or supplemented by, a reconstructed, revised concept, in the way for example, that concepts of heat, sound, force, and solidity have been refined or revised or supplemented by scientific concepts.

Caution is necessary, however. There appear to be two reasons why there might be a need for a reconstructed concept of colour. One could be that the natural concept needs to be eliminated. The other might be that it needs to be supplemented by a new, technical concept. Each reason is related to the fact that the natural concept serves a range of purposes. If it turns out that all, or nearly all, the major purposes are best served by a new revised concept then that would be a case for replacing the old concept. If only certain of the purposes are better served, then that constitutes a case for supplementing the old concept. Given that there is a natural concept of colour which is employed both in colour vision and in the many social practices concerned with colour, then it will be important to specify what the natural concept is, in order to appreciate how if at all it needs to be revised.

It is crucial to remember, moreover, that in the case of colour, almost all the purposes to do with colours and hence reasons for having concepts of colour have to do with the perception of colour. Colours function predominantly as natural and conventional signs, i.e., for various practical epistemological and social purposes. To the extent, therefore, that the natural concept needs to be revised or replaced by any reconstructed concept, the new concept(s) would need to be capable of serving those various purposes. Accordingly an examination of what the natural concept is will be vital for the justification of any theory about how colours should be conceived.