Embodiments provide a method for processing a first image based on a second image, wherein each pixel in the first image has a plurality of color components and has a corresponding pixel in the second image, and wherein each value of a color component in the first image corresponds to a value of a color component in the second image. The method may include deciding, for each color component of a pixel in the first image, whether to modify the value of the color component dependent on a predetermined criterion. The method may further include determining a similarity index between the pixel in the first image and the corresponding pixel in the second image based on, for each color component of the pixel, the value of the color component of the pixel, or if it is decided that the value of the color component of the pixel is to be modified, the corresponding value of the color component in the second image.