1. A method for generating digital colour values representing a test target from a colour negative image of the test target, recorded onto an image carrier, said test target including at least one grey swatch, comprising the following steps: scanning said colour negative image with a colour image scanner; separating the colour negative image into a plurality of colour components; converting each colour component into a plurality of electrical colour values, each colour value representing a portion of the colour negative image; establishing a relation between the plurality of electrical colour values of each colour component and a luminance of the at least one grey swatch; and, transforming said plurality of electrical colour values into a plurality of corresponding digital colour values according to said relation, said transforming step being done independently by one tone curve per colour component, derived from said relation; and, wherein the drafting of the tone curve for each of the colour components comprises the steps of: estimating the global brightness range of the test target as represented by the colour negative image; determining for every electrical colour value the corresponding luminance of the at least one grey swatch; associating to every electrical colour value a digital colour value to which is assigned; a minimum value if the corresponding luminance is lower than the lowest luminance in the global subject brightness range; a maximum value if the corresponding luminance is higher than the highest luminance in the global subject brightness range; and a value between the minimum and maximum values that is in a linear relation to the corresponding luminance if said luminance is lying within the global subject brightness range.