The determination of the colour gamuts of colour reproduction media is an important aspect of both understanding them in isolation and using them in the context of a colour reproduction system. Although this is well understood for output colour reproduction media, a solution for input media is not to be had in a simple way. To this end, in this article, we propose a method based on simulating the responses of an input medium to given spectral power distributions. We then determine the gamut of an input medium on the basis of having a set of spectra that covers the majority of all possible spectra, knowing a medium's quantized responses to them, and then determining a boundary beyond which the medium does not produce variation in its responses.