A method of matching image color and or luminance characteristics in an image processing system. In order to match an input image with a reference image, a color transformation M is initialised (601). An output image is copied (602) from the input image. The following sequence of operations is then repeated: Output and reference images are displayed on a system monitor. The user identifies (603) a highlight, shadow or overall region in both images. These regions are processed (604) to identify a difference (605). The difference is concatenated (606) onto transformation M. The output image is updated (607) by processing the input with M.