The problem of colour image segmentation is investigated in the context of mathematical morphology. Morphological operators are extended to colour images by means of a lexicographical ordering in a polar colour space, which are then employed in the preprocessing stage. The actual segmentation is based on the use of the watershed transformation, followed by region merging, with the procedure being formalized as a basin morphology, where regions are “eroded” in order to form greater catchment basins. The result is a fully automated processing chain, with multiple levels of parametrisation and flexibility, the application of which is illustrated by means of the Berkeley segmentation dataset.