The physicists' account of colour, both in its original and in its reconstructed version, depends on a certain view of the natural concept of colour. So too, in their differing ways, do the accounts of their most important critics. Getting clear about the natural concept of colour will thus be essential for resolving that philosophical debate. Quite apart from that, the natural concept is important to describe for its own sake. Getting clear about the natural concept of colour is necessary for a philosophical understanding of colour.

Discussions of what kind of property colour is are often driven by epistemological and metaphysical concerns, which can cause us to lose sight of what seem to be the plainest and most obvious truths about colours. Two of the most obvious truths have already been cited: (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. There might be a theory according to which these are not truths at all but only `truths'. Such a theory cannot be ruled out, but to be acceptable it would need to explain why the `truths' have the force that they do as apparent truths.

There is another truth that is more often ignored: that colours play significant roles in the epistemological, personal and social lives of human beings. Colours are important epistemologically, as natural signs or indicators for the identification and re-identification of physical objects. From a social point of view, colours serve a variety of purposes. While retaining their function as natural signs, they also serve as conventional signs, e.g., as badges, uniforms, in ceremony, ritual etc. They may also be said to have a `life of their own'. As well as having emotional and aesthetic effects, colours are used in social life to amuse, to entertain, to delight, to shock, to impress, to astound, to warn, to attract, to be enjoyed, and so on, in contexts having to do with pageantry, ceremonial, courtship, painting, lighting, plays, clothing, dining, drinking, and so on.

Recognising these obvious truths highlights the importance of the natural concept of colour, for it is the way in which colours play their various roles that the natural concept is significant. In the first place, colours play their roles through the exercise of colour vision, which in turn involves the exercise of colour concepts. The perceiver must have perceptual experiences, or acquire perceptual states, which have a certain content. It is through such experiences that colours serve as natural signs, i.e., for the identification and re-identification of objects. Some account needs to be given of what constitutes the content of these perceptual experiences or states, i.e., of what kind of property colour is presented or represented as being.

In the second place, there is a wealth of uses for colour in our cultural and social life, giving rise to a flourishing colour vocabulary. Examples of the sorts of practices are the use of colour language, i.e., in the use of predicates such as `blue', `red', `white' etc.; the teaching and learning of colour predicates, by the use of paradigm examples; the sorting and classifying of objects; the placing of colour samples in ordered structured arrays, the use of colours to impress, delight, astound, court, entertain, and so on. Central among such practices are those involving the use of colours as both natural signs or indicators, and as conventional signs.

If we concentrate on the use of colour predicates such as `red', `blue', `olive' etc., in natural language, it is possible to specify what we might call the `folk concept' of colour, one expressed by such terms. There is some advantage, however, in using the term `natural concept' to emphasise that the folk concept is built upon the use of a biological endowment, one that is exhibited in the use of colours as natural signs, for the identification and re-identification of physical objects. Whatever it is called, it is clear that, if we wish to give an account of the epistemological, personal and social roles served by colours, then we need to give an account of the natural or folk concept of colour, the concept which is embedded in the activities and practices that form the basis of such roles.

To specify the natural or folk concept of colour, therefore, requires studying the variety of activities and practices, linguistic and non-linguistic, in which colours play a role. To specify this concept is a central task for any theory of colours to perform. Colour is not just a topic for scientific experts. The ordinary folk are experts too. They have expertise in recognising colours, in sorting and classifying them, in using colours and in responding to them. Colour experts are not just those who study colour in a scientific way, nor those who paint in colours, nor those who are industrial chemists. There are, in other words, different ranges and levels of expertise. Those of us who are competent with colours know a lot: we know what colour blue is, how it differs from red and from yellow and green; we know how dark blue differs from light blue; we use terms such as rich, pale, faded, intense, brilliant, bright, pure, mixed, and so on to convey and exploit what we know.

Recognising the expertise of the `folk' should also make us alert to the dangers of using the term "the folk concept" of colour. The term `natural concept' is much safer to use. For one thing, `folk concept' gets easily conflated with `folk theory', i.e., some doctrine that we in our naive moments would articulate if asked, and if we had the time to reflect on. There are two things wrong with this slide. In the first place, the natural concept is a concept (or set of related concepts) not a theory (though its possession may presuppose certain beliefs). In the second place, by calling it a theory, it is easy is to over-intellectualise the concept. The concept is one embedded in a vast set of conceptual practices, engaged in by colour experts, those who are competent in the perception, recognition and use of colours. The knowledge is implicit as well as explicit, and it involves know-how besides.

In providing an account of the natural/folk concept of colour, there are two sorts of description that we can provide: (i) a description of the way colour is conceptualised, i.e., the kind of property colour is conceptualised as being; (ii) a description of the kind of concept the natural/folk concept is, i.e., a description of how the concept is acquired, how it is exercised, the purposes it serves, and so on. In this respect it is possible to describe the folk concept of colour as follows:

1.colour concepts are perceptual concepts;
2.colour terms such as `red', `green', `blue', etc are taught by the use of paradigm examples;
3. there is a distinctive colour vocabulary the terms of which are taught by paradigm examples.
4.it is through the exercise of colour concepts (through the way coloured objects appear), that colours fulfil their (practical) epistemological role: to serve as the signs for identification and re-identification of physical objects.
5.it is through the exercise of colour concepts (through the way coloured objects appear), that colours fulfil their social roles: to serve as conventional signs
6.it is through the exercise of colour concepts (through the way coloured objects appear), that colours fulfil their aesthetic and emotional roles.

It is through the study of the activities and practices which involve the acquisition and exercise of such concepts, that we can state certain principles about the kind of properties colours are conceptualised as being. These principles are implicit or explicit in the activities and practices. One such principle is that colours are perceptually salient, i.e are the sorts of properties that colour vision is sensitive to and which are presented or represented in perceptual experience. Other important principles are those having to do with fact that colours as a group form structured arrays, with characteristic internal structures.

This last feature is perhaps the most significant feature about colours, the colours objects are represented as having. Colours 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. Colours can be placed in systematically ordered arrays, along three dimensions, e.g., hue, saturation, and brightness. There are different arrays according to whether the colours are surface colours, film(aperture) colours, volume colours, light colours etc. Each array has a distinctive, complex character: a fourfold structure, based upon four distinctive, unique hues: green, red, blue and yellow. All colours can be mapped according to how near to or far from they are to any two of these unique, primary colours. Moreover, more than one such colour system can be constructed even for the one mode of appearance: the Oswald, Munsell and Swedish Natural Colour systems are examples. The last-named for example, uses dimensions of hue, chromaticness and whiteness/blackness, whereas the Munsell system uses hue, chroma and lightness. These dimensions are most suitable for surface colours, whereas hue, saturation and brightness are more appropriate for aperture colours.

In short, we can state that, given the natural/folk concept of colour, colours are the following kind of properties:

1. Colours are perceptually salient:
they are the sorts of properties that one discerns directly by looking, that one recognises the objects as having. they are qualities that are presented (or represented) in visual experiences. Visual experiences characteristically are experiences of colours, and of shapes and other qualities of objects in a 3-dimensional space.
They are properties causally connected with perception and identification of specific objects as red, yellow, blue, and so on.

2. Colours are the sorts of properties that can be arranged systematically in ordered arrays. That is, a set of internal relationships hold between the range of colours.

3. Colours comprise the kind of property which make true a wide range of first order colour principles or statements. Some examples are the following:
- tomatoes are red, so are sunsets and blood, and so on.
- red merges gradually into yellow in one direction, and into blue in another direction, but not into green except through either yellow or blue; and so on.
- ripening bananas go from green to yellow, certain ripening apples go from green to red, and so on.

The statements cited in 3 are meant to be illustrative examples. There is a considerable variety in the types of statement involved. Some are causal truths e.g., ripening pears and wheat go from green to yellow, acids turn blue litmus paper red, spiders with red stripes on the back are venomous, black hair tends to grow grey with age, and so on. In the second place, there are certain aesthetic and emotional facts. For many people, green is soothing. Soft pastels are suitable in certain contexts; mauves and browns are not. Light colours and dark colours have different effects on mood, and so on. Certain colours are harmonious, and others jarring. In addition, there are truths comprising the way perceived colours vary with illumination, distance, orientation, and so on. There are other truths concerning how the colour of surfaces is affected by the background against which an object is seen.

What has been just given is a partial characterisation of colours, given the natural or folk concept of colour. It is plausible to hold that a fuller characterisation than this minimalist version can be provided. Unfortunately any such account is likely to be controversial. Frank Jackson (1996) describes what he calls `a prime intuition' about colours that there is something peculiarly visually conspicuous about colours: "Redness is visually presented in a way that having inertial mass and being fragile, for instance, are not, When we teach the meanings of the colour words, we aim to get our hearers to grasp the fact that they are words for the properties putatively presented in visual experience when things look coloured." [p. 199.] This prime intuition, Jackson states, is simply that red is the property objects look to have when they look red. This deceptively simple prime intuition is said to tell us something important about the metaphysics of colour when we combine it with plausible views about what is required for an experience to be the presentation of a property: a necessary condition for experience E to be the presentation of property P is that there be a causal connection in normal cases. [p. 200.]

The idea behind talk of `prime intuitions' is that they are the intuitions formed by those competent in the use of colour language, especially in the identification and recognition of colours. Those competent perceivers and users of colour language are persons capable of reflecting on the way in which colours are presented (or represented) in colour experience. Accordingly, it is plausible to hold that colours are presented as follows: as objective, perceiver-independent, intrinsic features of physical bodies, i.e., physical surfaces, volumes, light sources, illuminations, films, media, and so on.

The expression of this intuition adds little to the previous minimalist characterisation of the folk/natural concept of colour. It is plausible to go further and hold that colours are not only intrinsic features of physical bodies, but are presented as manifest, sensuous properties. The way they are manifest is that their nature is open and manifest, not hidden. Some philosophers are prompted to respond to this claim by saying that it begs the question against those theorists who hold that colours have hidden essences, e.g., physical microstructures. The counter-reply to this response is that the view that colours have hidden essences is not the right theory about the natural concept or folk concept of colour. It may be plausible as a theory of what a reformed concept of colour is or should be, but not as a theory of what the folk concept is.   Admittedly it is not easy to convince people that there are manifest properties. One of the problems is that many concepts begin their life as concepts of manifest properties, but then evolve into more complex concepts. Children's concepts of say `horse', `dog', `man' and so on, are cases in point. For them, the concept is almost exclusively defined by the corresponding appearances. There are, however, more sophisticated examples. Take concepts such as brilliant, dazzling, sprightly, po-faced, cheery, glum, picturesque, grim-faced, pale, etc. These are terms that characteristically apply to appearances. All of them, it is plausible to suggest, are manifest properties with essences they `wear on their faces', and are not hidden.

Many properties are not manifest: being poisonous, being made by a robot, containing water as a constituent, coming from Virginia, and so on, but some clearly are. Included among these are colours. Someone who is taught colour terms and who understands how they are used, knows what it is for something to be red, to be blue, or whatever: it is to have that feature which the perceiver is capable of recognising. Reflecting on the way colours are represented, the thoughtful observer can say that the sort of property colours are represented as being are as colour `stuffs' spread on the surface of physical bodies (or through volumes, etc.). They are intrinsic features of physical surfaces (volumes, . . .), spread over the surface. It seems only too clear that we experience the redness of a ripe apple as an objective quality of the apple, the redness being in an objective space just as much as are the shape, the contour, the texture of the apple. This point can be neatly illustrated by quotes from two eminent workers on the physiology/psychology of colour, Hering and Boynton. Hering for example, writes:

"When we open our eyes in an illuminated room we see a manifold of spatially extended forms that are differentiated or separated from one another through differences in their colours . . . Colours are what fill in the outlines of these forms, they are the stuff out of which visual phenomena are built up; our visual world consists solely of differently formed colours; and objects, from the point of view of seeing them, that is, seen objects, are nothing other than colours of different kinds and forms."
[Hering (1964), p. 1]

In similar vein, the physiological psychologist Richard Boynton writes in `Colour in Contour and Object Perception': "From early childhood we are easily able to recognise a property of objects, usually associated with their surfaces, that we call colour. No child, and relatively few adults, will doubt that colour is on (or sometimes in) objects." [Boynton (1978), p. 175] In addition, one is aware of the different character of the way colours appear in different modes, i.e., for object surfaces such as apples, patches of light on screens, volumes such as wine, scattering media such as skies, light sources such as globes, and so on.

There is one more prime intuition which is one of the most important. It is part of the folk concept, another `prime intuition', that colours are represented as qualitative, sensuous features. This point will no doubt be controversial, but it ought not be. Reference to the sensuous nature of colours is crucial. These qualitative features that colours are represented as being are `sensuous' in the widest sense. This is not an issue of deep metaphysics. The term "sensuous" is often used on such a way as to apply to phenomenal, i.e., to ontologically subjective qualities. However, there is a wider sense which does not have this commitment. There is a neutral use for the term. An illustration is an example which H.H.Price borrows from Husserl: `When I see a tomato hanging on a vine then a ripe tomato hanging on a vine is "leibhaft gegeben": it is given to me with its sensuous qualities.' [Price (1932), p. 231.] This sound much better, of course, in German, but in English the point is that the tomato (better still, grapes) and the vine are given in perception with the sensuous features. English speakers understand that in perceiving tomatoes, grapes, etc one is acquainted with sensuous features. Price was acknowledging that whatever one's theory of perception, and especially whether one thought that the perceiver is directly aware of physical objects or sensory presentations, one was acquainted with sensuous features.

A similar point is made by Evan Thompson (1995) though with respect to the term `phenomenal', rather than `sensuous'. Research in psychophysics and visual physiology, he writes, is constrained by the `phenomenal structure of colour'. By this term he means to refer primarily to the three dimensions of colour, known as hue, saturation and lightness, as well as to the relations that colours exhibit among themselves (p. 39). As he points out, textbooks often classify these properties of colour as `subjective colour phenomena' or as features of `colour experience'. Thompson prefers to use the term `phenomenal' to describe them because they are first and foremost features of how colours appear: "I thus intend to use the term `phenomenal' in its older sense of pertaining to appearances, not in the current sense of subjective."

The neutral notions of `sensuous' and `phenomenal' are ones that can be shared by writers with very different philosophical commitments. It is such a notion that Michael Tye employs, when he states that when philosophers appeal to the phenomenology of perceptual consciousness, in making claims about the phenomenal character of experience, they are mistaking intrinsic features of the content of experience for intrinsic features of experience itself. Tye considers a hypothetical appeal to first person phenomenology:

        Standing on the beach in Santa Barbara a couple of summers ago on a bright sunny day, I found myself transfixed by the        intense blue of the Pacific Ocean. Was I not here delighting in the phenomenal aspects of my visual experience? And if I        was, doesn't this show that there are visual qualia? [Tye (1992), p.160]

 He is not convinced. It seems to him

        that what I found so pleasing in the above instance, what I was focussing on as it were, were a certain shade and intensity        of the colour blue. I experienced blue as a property of the ocean not as a property of my experience. My experience itself        certainly wasn't blue. Rather it was an experience that represented the ocean as blue. What I was really delighting in then        were specific aspects of the content of my experience. It was the content, not anything else, that was immediately accessible        to my consciousness and that has aspects I found so pleasing. [Tye (1992) p. 160]

Tye's point, it would seem, is that philosophers are wrong who attribute to perceptual experiences phenomenal character. Instead they ought to attribute this character to the content of the experience. Tye implies that in visual experience we experience objects such as oceans as having phenomenal character. This is the same thought as expressed in this article: our visual experiences have certain content, i.e., we experience objects as having sensuous qualities. It is part of the concept of colour that is the content of visual experiences that colour is a sensuous, objective feature of objects.

Accordingly, we can represent writers as diverse as Price, Thompson and Tye, despite their philosophical differences, as in agreement. There is a neutral sense of `sensuous', or `phenomenal', according to which it is possible for physical objects to have sensuous or phenomenal properties. Most importantly, the colour properties that the natural concept of colour attributes to physical objects are sensuous properties. It is of course a separate question of whether physical objects do have the sensuous features that they are represented as having. Price thinks that they do not, but he also thinks that a further argument is required to show that they are not.

To conclude: given the characterisation of the natural concept of colour, colour is 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.