A report is presented of computer simulations which demonstrate the applicability of self-organization principles to the formation of a cortical colour map. A two-dimensional array of cortical units can be shown to become selectively sensitive to different colour stimuli in an orderly fashion. The precortical part of the simulation model contains three wavelength-sensitive receptors with overlapping sensitivity distributions, and a simple opponent processing stage. Each cortical unit receives the same activity from the precortical stage by adaptive connections. Initially the connections are arbitrary; during self-organization they are changed so that different way. Self-organization of the connections can be shown to take place even when the stimulation of the receptors is totally random, ie the wavelength and purity (and in some simulations the distribution) of each stimulus are selected randomly.