1. A non-transitory, computer storage media having stored thereon a sequence of program instructions that are executable by a computer system to perform a method of modifying pixel data in a target image, by: storing a target image of an object with one or more pixels having spectral data to be modified in a memory of the computer; storing a source image of the same object in a memory of the computer; classifying pixel spectral data in the source image to identify boundaries of local class areas each including a number of neighbouring pixels having similar spectral data values; determining boundaries of local class areas in the target image that correspond to the boundaries of the local class areas in the source image; determining a pixel in the target image having spectral data to be modified; identifying a replacement pixel in the source image having spectral data that will replace the spectral data for the pixel in the target image; identifying the boundaries of one or more additional local class areas in the source image that have groups of neighbouring pixels with spectral data that are classified in the same manner as the spectral data in the local class area that includes the replacement pixel; identifying the boundaries of one or more additional local class areas in the target image that correspond to the one or more additional local class areas in the source image; determining one or more scaling relationships between the pixel spectral data in the boundaries of the one or more additional local class areas in the target image with the pixel spectral data for pixels in the corresponding one or more additional local class areas in the source image; and replacing the spectral data for the pixel in the target image having spectral data to be modified with the spectral data for the replacement pixel in the source image using the determined one or more scaling relationships.