The natural or folk concept of colour conceptualises colour as a certain kind of property: an objective, perceiver-independent, intrinsic feature of physical bodies. The property is, moreover, one with a certain character and with certain causal properties. The most straight-forward view of colours is that they are what they seem. Colours are properties whose essences are manifest. Since colours here are taken to be simple, primary quality of objects, we might call this view "The Simple Objectivist View of Colour". It stands in clear contrast to those forms of Objectivism according to which the essences of colours are hidden, not manifest. [See OBJ].

Opposed to both forms of Objectivism is another view: `The Illusion Theory of Colours'. In this account, it is held that once we spell out the character of the features specified by the folk or natural concept, we discover that there is in nature no such features: colours as they are conceptualised are properties not found in nature. The colours objects are represented as having, in visual experience, are ones that no object actually has.

The natural concept of colour conceptualises colour as a certain kind of property. Which kind it is can be specified, in part, by saying that it is an objective, perceiver-independent, manifest and sensuous kind. In addition the property is one with certain kinds of causal powers vis a vis the presentation of colour in the perception, recognition and identification of colours. Finally, colours are the kinds of properties that fit together in characteristic ways to form structured colour arrays, with a distinctive 3-dimensional character. They are properties that as a group, form an internally related 4+2 structure, built on the four unique, primary hues: green, red, blue and yellow, and related to the black/white pair. Some parts of this characterisation of the natural concept are contentious, e.g., the claims that colours are manifest and sensuous. Some of the most significant parts of the characterisation which have the most far-reaching implications are not controversial: that colours have causal powers as described above, and that collectively form a structured system.

Defenders of the Illusion Theory of Colour exploit the presence of these features in the natural concept to argue that, given this concept of colour, there are in fact no colours in nature, that objects are presented in experience as having colours which neither they nor any object have. Crucially, there are no properties that both have the causal powers in question and which collectively have the right character. In short, there are no colours that are intrinsic, non-relational, perceiver-independent properties and which satisfy the requirements of the three-dimensional colour solid. None that is, that allow us to make sense of the way in which we perceive and identify and recognise colours.

No properties of physical objects stand, it has been said, in the right kinds or relations that are characteristic of the structured colour arrays. It is true that we can arrange physical samples in ordered arrays but the ordering principles depend on the way they appear. What is crucial to the principle of ordering is the way the colours are represented as being, or rather, the character of the way colours are represented. It is because there is a distinctive appearance associated with each colour that the colours are capable of being systematically ordered in the way that they are.

It would seem that, as far as our conceptual practices governing colour are concerned, physical objects do not have the kinds of colour they are represented as having. The colours that objects are represented as having are illusory: no physical object actually has those colours. The colours might be said to be "virtual properties": they are properties objects do not have, but might have had: in some other possible world but not in this one. If we speak of colours as having essences, then they have virtual essences. Colours are virtual properties, just as phlogiston and caloric are virtual natural kinds.

The illusion theory, or virtual essence theory, of colours leaves us with a problem. If there are no properties that satisfy the requirements for being colours: how did the natural concept develop? The solution to this problem is found in the fact that the way that the concepts of colour operate, to serve their various functions and roles, is through the way colours appear. For these purposes and roles, objects do not need the actual colours. It will be sufficient if they appear to have colours. For these purposes, it is sufficient that "it is as if they have the colours".

There are two major functions for colour concepts. One reflects an epistemological purpose: colours are signs used to indicate the presence of objects of interest. The signs are either natural or conventional, the latter being ones designed for various social purposes. The purposes are equally well served even if objects do not have colours, but have the right appearances. All that is needed is that they are represented as having colours. The second major purpose of colour concepts is aesthetic, understood in the widest sense. Colour is significant in painting, decorating, clothing, theatre, make-up, advertising, showing off, sexual appeal and so on. Again, it matters not in the least that objects do not have these properties. All that is required is that they be represented as having them.

The significance of appearances is widespread. As we have seen, they provide the basis for the ordering principles governing colour systems. Likewise, the causal truths and principles that employ colour terms are ones connected with the way colours appear. For example, we can explain how there are such `truths' as "ripening pears (bananas, . . .) go from green to yellow". This is a truth. For a pear to be represented as green under the right conditions is a sign that the pair is not yet ripe; for it to be represented as yellow is a sign that it is ripe. In other words, whatever causal truths we have concerning colour, can be explained by interpreting colours as signs or as indicators for other physical features, where those physical features serve the causal roles.

In a previous section a distinction was made between colour science and the science of colour. While the former field is flourishing there is little science of colour. One way of understanding how this situation has arisen is that there are no actual colours in physical reality. What there are are experiences which represent objects as having colours, colours which in fact they do not have. That is, colours are virtual properties. Our visual experiences present us with systematic illusions. If this were the case, we would still have the same colour science, exactly as we have now, for we would still need to know how colours are represented, and what causes them to be represented in the way that they are, and how the various conditions under which we have colour experiences systematically differ can be explained. Since one of the central roles colours serve is to act as signs or indicators for physical objects, and any theory of colour has to acknowledge this role anyway, it would seem that any fledgling science of colour is best dispensed with in terms of other sciences, and colour science left to the science of colour perception. This does not stop it from being the case that there is an important theory of colour vision and perception, and of the role colours play as signs or indicators.

Clearly, the concept of colour can be used to serve many of its normal purposes even if the representations of colour are illusory, providing that the illusions are systematic, which on a proper theory, of course, they will be.