Major opposition to the objectivists come from those philosophers who hold that colours are essentially mind-dependent dispositional properties: powers to appear in distinctive kinds of ways to perceivers of the right kind. Such accounts are often called `subjectivist'. In comparing the merits of objectivist and subjectivist accounts, it is helpful to study the examples of solidity and liquidity, for these examples provide a set of parallels for thinking about colour.

In the case of solidity and liquidity, there is a range of causal capacities that historically have been thought to be constitutive of these properties. The growth of science has seen the discovery of the microstructural properties that form the causal ground of these capacities and powers (at least in broad terms). This discovery does not mean that such microstructures constitute the essences of solidity and liquidity. There are at least two ways of thinking about what solidity and liquidity are essentially. On one model, solidity is essentially the microstructural property and the description of the causal powers forms an essential part of the way the property is characterised, rather than an essential part of the property itself. There is a second way of thinking about solidity, according to which solidity is not identified withe microstructural basis even if the latter is unique. Rather the causal powers are essential to solidity, either because solidity is identified with them, or because solidity is taken to be a second order property: for something to be solid is for it to have some property which is the basis for the relevant causal powers.

As far as colour is concerned, it would seem that the objectivist would need to depend on either of the last two models. For most objectivists take colours to be essentially dispositional properties, ones characterised in terms of reflectance profiles. An object's reflectance curve represents a dispositional property: a power to differentially absorb or reflect light from the range of wavelengths constituting daylight (or a standardised equivalent).

It would seem, therefore, that as far as revisionary accounts of colour are concerned, the choice is between different kinds of dispositionalist accounts: objectivist and perceiver-dependent (subjectivist). On both analyses, what colours are essentially is given by a description of appropriate causal powers. In one case these causal powers are objective ones; in the other, they are special perceiver-dependent causal powers. Let us call the respective kinds of colour `objective colour' and `psychological colour'. In assessing these accounts as providing revisionary proposals, the important question to ask is what can be achieved by adopting the respective proposals. There is reason to think that psychological colour is superior, or at least as good.

The psychological, i.e., mind-dependent property is usually presented as being a pure disposition: to call something `red' is to say that it has the power to appear red to observers of the appropriate kind. A far better proposal, which can be found in the writings of Descartes and Locke, is one that presents colour in terms of a mixed disposition:

x is red = x has some feature by virtue of which x appears red, . . .

This concept has all the advantages of the objectivist concept, and added virtues of its own. It allows for multiple realisations of the disposition, and hence of the colour. It does not require that for each colour there is a unique physical basis. Second, by placing emphasis on appearances, it provides the means to unite the various kinds of colour: surface colour, volume-colour, aperture colour, illumination-colour, etc. And finally, it can perform the one function that the physical concept does very well: it shows how colours can have a causal role in relation to the perception of colour, and the social roles played by colours.

That the mixed dispositionalist account readily solves the problem of multiple realisations is obvious. It is one of the central advantages of the psychological account, however, that it both provides a connecting link between all the various kinds of colour: surface colour, volume-colour, aperture colour, illumination-colour, etc., in that it unites them all as colours while at the same time it makes intelligible their differences. For each of these colours, there is a distinct mode of appearance. For each mode of appearance, colours can be organised into systematic 3-dimension colour arrays. And for each array, hue is one of the dimensions: colours can be ordered with respect to how close they are to red, green, yellow and blue. There are, however, important differences. For aperture colours, the other dimensions are saturation and lightness. for surface colours, they are chroma and value, or chromaticness and blackness/whiteness. As well, the greens, blues, yellows, etc of surfaces are different types of green, blue, yellow, etc from those of films _ but they are greens, blues, yellows, etc, for all that. It is through characterising surfaces, films, illumination-sources, and so on, as providing appearances that sense can readily be made of the range of similarities and differences between the various kinds of colour. The point of having a dispositional concept framed in terms of the way things appear is that it helps provide principles of unity and diversity for the available range of colour systems.

Finally the mixed dispositional concept can perform the same function that the objectivist concept can serve: it can be used to show how colours are causally relevant to the perception of colour. The mixed dispositional concept retains the emphasis on red objects having the right kind of power, but it allows that the object, in having that power, has some physical feature (which may be different in different objects) which is the basis for that power. Clearly, on this analysis, the underlying physical feature has all the causal powers one could wish. The mixed dispositional analysis combines this with the advantage of keeping colours tied to the way they appear.

In addition, there is reason to think that coloured objects appear is an essential part of the range of conceptual practices. The point here is that while reflectances are causally relevant, as for that matter are microstructures, so too are appearances. In the case of colour there is a deeply entrenched set of activities and practices central to which is the operation of causal powers to appear, i.e., powers to cause perception of objects as red, blue, etc. These causal powers are also central to the field of colour science. Given this twofold fact, then if we are considering a revisionary concept of colour, then, by analogy with solidity, there is good reason to propose a dispositionalist concept of colour, for which the power to appear in a way distinctive for individual colours is essential.

The important point, as far as colours are concerned, is that coloured objects have characteristic appearances and that those appearances are of great interest to us. It is because we have that interest that there is point to having a concept of dispositional colour - the power to appear in characteristic ways. It is because of the way colours appear that they are important to us both biologically and socially. It is because colours have a characteristic appearance that: the colours can be ordered systematically in colour arrays; they have emotional effects; principles of harmony and contrast apply; there are principles governing phenomena of colour contrast. It is true that physical features both of physical objects and of retinal cells contribute causally to these phenomena, but central to all of these colour principles is the way colour appears.

At this stage, an objectivist might argue that there is a more fundamental causal power, one associated with reflectance curves, and for this reason it would be preferable to adopt, as a revisionary proposal, a concept of colour whereby this more fundamental causal power is essential. If these two proposals are seen as competing theories, it is not clear that one is preferable to the other. Moreover it is not clear why we could not adopt both proposals and have two concepts of colour, just as in the case of the geometrical property, size, we have two concepts: absolute (intrinsic) size, and angular size. That this ecumenical solution represents a viable option receives support from the consideration that if we are thinking of the proposal for the objectivist concept we have at least two proposals: one framed in terms of intrinsic microstructural properties, and one framed in terms of dispositional light-related properties. Again rather that having to choose between them, we could adopt both proposals and admit that there are different kinds of colour.

In conclusion, therefore, it would appear that in so far as the objectivist is offering us a reconstruction of our original concept of colour, there is reason to think that a dispositional analysis would provide a construction that is at least as good. There is strong reason, however, to think that an ecumenical solution to the problems of colour can be found: that there is a place for different concepts of colour, and with them different essences.