Two light beams that are seen of the same colour, but possibly having different spectra, are said to be metameric. The colour of a light beam is based on the outputs of a set of photodetectors with different spectral responses, and metamerism results when such a set of photodetectors is unable to resolve two spectra. Metamerism can be characterized in terms of the L, M and S responses of the receptoral layer in the retina, or in terms of the CIE X, Y and Z curves, etc. Working with linear spaces, metamerism is mathematized in terms of the kernel of a certain linear transformation; we derive a basis of localized support functions for such a kernel. We consider metamerism in general and we consider metamerism in the trichromatic and tetrachromatic cases, in particular. Applications are in computer vision, computational photography and satellite imaginery, for example. We make a case for hue metamerism, where the luminance and saturation of two colours of the same hue may be different.