Abstract: Is it possible that a person who behaves just like you and me in normal life situations and applies colour words to objects just as we do and makes the same colour discriminations and colour similarity judgements that we make, see green where we see red and red where we see green? Many philosophers assert that the description of such a case is somehow incoherent. Often the motivation for this assertion is “that they suspect that admitting that claim [the possibility of such a case] will put one on a slippery slope which will eventually land one in skepticism about other minds”.1 Among philosophers, however, it does not seem to be common knowledge that there is scientific evidence for the existence of such cases. Theories about the physiological basis of colour vision deficiencies together with theories about the genetics of colour vision deficiencies lead to the prediction that some people are ‘pseudo- normal’ (according to an estimation of Piantanida (1974) this occurs in around 14 of 10 000 males). 2 Pseudonormal people “would be expected to have normal colour vision except that the sensations of red and green would be reversed – something that would be difficult, if not impossible, to prove. ”3 Any philosophical theory of mind or more specifically about colour, colour appearances or colour concepts should meet the following plausible prima facie constraint: No hypotheses accepted or seriously considered in colour vision science should be regarded according to a philosophical theory to be either incoherent or unstatable or false. Therefore – regardless of whether the hypothesis of the existence of pseudonormal people is correct- the mere fact that the hypothesis is seriously considered in colour vision science, is philosophically relevant. Central claims of colour vision science when combined with specific empirical assumptions lead to the prediction that there are red-green-inverted people. Therefore any philosophical theory which excludes such a case does not meet the above formulated constraint. The failure to meet this prima facie constraint does not in itself justify the rejection of a philosophical proposal, but it does represent a serious objection. This kind of criticism will be advanced against some widely held philosophical proposals in the present paper. But let me begin with a short sketch of the relevant parts of colour vision science.