1. Apparatus for processing image data, including memory means for storing image date and processing instructions, processing means for performing said image processing, said instructions defining operations to be performed by said processing means in order to process said image data, said processing means being configurable by said instructions such that: an output image is to be matched with a reference image, and a matching transformation has been initialised; and said operations include the step of: color sampling pixels in a user-identified region in said output image for matching with a region in said reference image; and repeated steps of: color sampling pixels in a user-identified region in said reference image; updating said matching transformation in response to a color difference between said user-identified regions; and updating said output image by processing image data with said updated matching transformation.