Ink set optimisation

A method of selecting optimal inks from a plurality of ink candidates for a given colour is disclosed. The method comprises a) obtaining a measure of spectral reflectance of a print substrate; b) obtaining a measure of the spectral reflectance of a first candidate ink of a first colour; c) predicting a colour gamut for the first candidate ink based on the spectral reflectance of the ink and the spectral reflectance of the substrate; d) repeating steps b) and c) for a second ink candidate of the first colour: e) selecting the ink candidate for which the predicted colour gamut includes the most target spot colours as the optimal ink for the first colour. The method may be employed to select optimal ink sets for an Expanded Colour Gamut (ECG) printing process. The substrate may be paper, a textile, a synthetic foil or a metal plate.