Data on the psychological structure of colour space was gathered from a dictionary of colour terms and by collecting responses to questions of the form, "which is the more similar pair: A and B or C and D" and "which is lighter: A or B?" Where A, B, C, and D were drawn from the eleven Basic Colour Terms. Analysis showed that the data possessed an approximate symmetry corresponding to the exchange of Red and Orange, Blue and Green, Purple and Yellow Black and White, Pink and Brown, and "darker" and "lighter." Analysis of a physical model of colour vision showed that a similar symmetry exists in the structure of the space of possible colour sensations. (C) 2001 John Wiley & Sons, Inc.