A method, comprising: receiving first image data relating to an unknown colour sample, colorimetry data for which is to be determined; receiving second image data relating to a plurality of known calibration colour samples, colorimetry data for which are already known; determining a plurality of colour calibration characteristics relating colour measurements of the known calibration colour samples from the second image data to the corresponding known colorimetry data of the calibration colour samples; and calculating colorimetry data of the unknown colour sample in dependence on colour measurements thereof from the first image data and the determined colour calibration characteristics; the method being characterised in further comprising compensating for brightness differences across the set of known calibration colour samples in the determination of the plurality of colour calibration characteristics, wherein the compensating comprises determining a first set of one or more functions having a first set of calibration coefficients, the one or more functions relating measured colours of the known calibration colour samples from the second image data to the known colorimetry data of the calibration colour samples and the known position of each known sample in the image; and analysing the determined functions to find a second set of functions having a second set of calibration coefficients, wherein the first and second sets of functions and calibration coefficients are used in calculating the colorimetry data of the unknown colour sample; wherein receiving first image data comprises measuring R, G, and B of the unknown colour sample and its position (x,y); receiving second image data comprises measuring for each sample i R i, G i, and B i at (x i, y i) position in the image and wherein the known colorimetry data are the corresponding XYZ values of each known calibration sample i X i, Y i, and Z i; and wherein the first set of calibration coefficients are found by determining respective relationships that map known X to measured R, taking into account the (x,y) position of each measured R, known Y to measured G, taking into account the (x,y) position of each measured G, and known Z to measured B, taking into account the (x,y) position of each measured B. receiving first image data relating to an unknown colour sample, colorimetry data for which is to be determined; receiving second image data relating to a plurality of known calibration colour samples, colorimetry data for which are already known; determining a plurality of colour calibration characteristics relating colour measurements of the known calibration colour samples from the second image data to the corresponding known colorimetry data of the calibration colour samples; and calculating colorimetry data of the unknown colour sample in dependence on colour measurements thereof from the first image data and the determined colour calibration characteristics; the method being characterised in further comprising compensating for brightness differences across the set of known calibration colour samples in the determination of the plurality of colour calibration characteristics, wherein the compensating comprises determining a first set of one or more functions having a first set of calibration coefficients, the one or more functions relating measured colours of the known calibration colour samples from the second image data to the known colorimetry data of the calibration colour samples and the known position of each known sample in the image; and analysing the determined functions to find a second set of functions having a second set of calibration coefficients, wherein the first and second sets of functions and calibration coefficients are used in calculating the colorimetry data of the unknown colour sample; wherein receiving first image data comprises measuring R, G, and B of the unknown colour sample and its position (x,y); receiving second image data comprises measuring for each sample i R i, G i, and B i at (x i, y i) position in the image and wherein the known colorimetry data are the corresponding XYZ values of each known calibration sample i X i, Y i, and Z i; and wherein the first set of calibration coefficients are found by determining respective relationships that map known X to measured R, taking into account the (x,y) position of each measured R, known Y to measured G, taking into account the (x,y) position of each measured G, and known Z to measured B, taking into account the (x,y) position of each measured B.