1. A computer implemented method for determining an output colour value for a component spectrum of a decomposed multispectral or hyperspectral image, the method comprising: determining or accessing the component spectrum and a first colour value for the component spectrum; receiving an input colour value for a pixel location in the image, wherein the component spectrum contributes to the pixel; determining the output colour value for the component spectrum based on the first colour value for the component spectrum, the input colour value and a utility score; and storing the output colour value for the component spectrum on a datastore.