The relations among consciousness, brain, behavior, and scientific explanation are explored in the domain of color perception. Current scientific knowledge about color similarity, color composition, dimensional structure, unique colors, and color categories is used to assess Locke's “inverted spectrum argument” about the undetectability of color transformations. A symmetry analysis of color space shows that the literal interpretation of this argument – reversing the experience of a rainbow – would not work. Three other color-to-color transformations might work, however, depending on the relevance of certain color categories. The approach is then generalized to examine behavioral detection of arbitrary differences in color experiences, leading to the formulation of a principled distinction, called the “isomorphism constraint,” between what can and cannot be determined about the nature of color experience by objective behavioral means. Finally, the prospects for achieving a biologically based explanation of color experience below the level of isomorphism are considered in light of the limitations of behavioral methods. Within-subject designs using biological interventions hold the greatest promise for scientific progress on consciousness, but objective knowledge of another person's experience appears impossible. The implications of these arguments for functionalism are discussed.

The qualitative and quantitative properties of color sensations and neuronal color coding are discussed in relation to physiological color exchanges and their evolutionary constraints. Based on the identity mind/matter thesis, additional physical measurements on color sensations are described that will allow us, at least in principle, to compare the qualitative properties of color sensations in different brains.

It is worth considering whether particular behavioral measures from observers are ever consciously (or preattentively) transformed a priori so as to render inferences about them indistinguishable. This is unlikely, but recent experiments indicating color sensitivity and selectivity without visual awareness suggest that the distinction between what can and cannot be explained about color experience using behavioral responses may not be as obvious as Palmer concluded.

There is reason to believe that people of different gender, race or age differ in spectra that are shifted relative to one another. Shifted spectra are not as dramatic as inverted spectra, but they can be used to make some of the same philosophical points.

Palmer argues that functionalist accounts of the mind are radically incomplete in virtue of a “metaphorical brick wall” that precludes a complete treatment of qualia. I argue that functionalists should remain unmoved by this line of argument to the effect that their accounts fail to do justice to some “intrinsic” features of experience.

Ignoring consciousness, I apply Palmer's ideas to a photometer, for which calibration is analogous to socialization of humans to agree in color. Some attributes of the photometer – such as its aperture – do not need to be known because their values are transparent to calibration. But a writing demon can wreak havoc if it permutes measured values before interpolation completes calibration – as happens in Palmer's color rewirings.

Palmer's “subjectivity barrier” seems to be erected on a popular but highly suspect conception of visual experience, and his “color room” argument is invalid.

Palmer's strategy of saving functionalism by constraining spectrum inversions cannot succeed because (1) there remain many nontrivial transformations not ruled out by Palmer's constraints, and (2) the constraints involved are due to the contingent makeup of our visual systems, and are therefore not available for use by functionalists.

Because, as Palmer shows, the only kinds of differences that can be detected are differences in relational structure, and relational structure is precisely what is preserved by isomorphism, his own arguments can be used to expose the incoherent motivation behind the traditional idea of “intrinsic qualities” of experience.

Palmer's suggestion that color might serve as an appropriate field in which to test the hypotheses on the relations among brain, mind, and behavior is misleading. Human color experience as related to mind must be considered as a cultural, as well as a natural phenomenon. It is important to avoid approaching facts of language as mere nomenclature; otherwise, concepts are reduced to percepts.

The quality of potential symmetries of the similarity structure of the Basic Colour Terms has been assessed. The assessment was made on the basis of a database of similarity judgements, made by subjects in response to linguistically expressed questions. All potential symmetries can be statistically rejected, although the well-known and some novel interpretable symmetries are shown to be approximately correct.

Color-order systems highlight certain features of color phenomenology while neglecting others. It is misleading to speak as if there were a single “psychological color space” that might be described by a rather simple formal structure. Criticisms of functionalism based on multiple realizations of a too-simple formal description of chromatic pheno-menal relations thus miss the mark. It is quite implausible that a functional system representing the full complexity of human color phenomenology should be realizable by radically different qualitative states.

Palmer's “isomorphism constraint” presupposes the logical possibility of two qualitatively disparate sets of sensory experiences exhibiting the same relationships. Two arguments are presented to demonstrate that, because such a state of affairs cannot be coherently specified, its occurrence is not logically possible. The prospects for behavioral and biological science are better than Palmer suggests; those for functionalism are worse.

The isomorphism between relational structures advocated by Palmer corresponds quite closely to Paul Churchland's theory of “state-space semantics,” so much so that one can be used to elucidate problematic areas in the other. The resulting hybrid shows eliminative materialism to be superior to functionalism as a theory of mental phenomena and seems to provide the best ontology for cognitive science.

Although the author's critical view of functionalism has a considerable intuitive pull, his argument based on the color room scenario does not work. Functionalism and other relational views of the mind are capable of providing coherent accounts of conscious experience that meet the challenge set up by the “color room argument.” A simple example of such an account is presented.

We discuss the difficulty of measuring the perceptual experience of colour, supporting Palmer's assertion that neuropsychological disorders of colour processing can be informative in this respect. We point out that some disorders seem to affect the perceptual experience of colour over and above the perceptual processing of colour, providing direct insights into the neural mechanisms supporting perceptual experience.

PURPLE (RED-and-BLUE) is the most frequently occurring derived (binary) basic color term (BCT), but there is never a named composite BCT meaning RED-or-BLUE. GREEN-or-BLUE is the most frequently named composite color category, but there is never a BCT for the corresponding derived (binary) category CYAN (BLUE-and-GREEN). Why?

Palmer's attempt to dust off Locke's construct of “inverted spectrum” is discussed here to examine its plausibility. Perceptual inversion could be fulfilled by adopting the notion of “inverted trichromacy” rather than by the proposed existence of “red-green reversed trichromats.” Although the former alternative conforms to a hypothetical world of vampires, it fails to conform to the realities of genetics and neuroscience.

Palmer describes a “subjective barrier” that limits knowledge of others' experience. We discuss how this barrier extends to all knowledge, becoming less distinct as theoretical constructs are strengthened. We provide evidence for isomorphic experience, among individuals with similar physiologies, by showing that perceived relations between colors are as similar when viewing pigments as when viewing subjective colors caused by flickering bars.

Purest points of Hering's six primary colors reside at different levels of lightness such that inversion of each hue pair would be detectable in subjects' choice of foci on the Munsell array. An inverted spectrum would not impose the isomorphism constraint on a contrast of red-green or yellow-blue, whatever we conclude about inference in functionalism.