Colour names are generally attributed to specific colour categories, of which the most representative colour is termed a focal colour. The question as to what underlies this categorization is addressed in this paper. Are the categories simply a naming convention of colours with a specific ‘perceptual salience?’ Results based on the wraparound Gaussian model of reflectances show that colour categories follow naturally from a perception-agnostic segmentation of colours defined in this space. In other words, a segmentation based only on colour coordinates, not perceived colours. Furthermore, it is demonstrated that the concept of a focal colour can be explained as a colour that is both (i) representative of a colour category, and (ii) relatively stable under a wide range of illuminant spectra without depending upon chromatic adaptation.