The colour gamuts of colour reproduction media are important properties of them and can play a decisive role in their use in colour reproduction applications as well as the improvement of their capabilities. While this topic has frequently been studied and is well understood and for output colour imaging media, a solution for input media is not to be had in a simple way. To this end, the present paper proposes a method for obtaining the theoretical gamut across which a device can capture colour differences and this method is based on simulating the responses of an input medium to given spectral power distributions. The gamut of an input medium is then determined on the basis of having a set of spectra that cover the majority of all possible spectra, knowing a medium's responses to them and then determining a boundary beyond which the medium does not produce variation in its responses. The present paper is an abridged version of a more extensive treatment of the topic submitted for journal publication.