1. A method for processing a first image based on a second image, each pixel in the first image having a plurality of color components and having a corresponding pixel in the second image, 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 comprising: 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; and 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, wherein deciding whether to modify the value of the color component is dependent on a comparison of an exposure level of the color component of the pixel in the first image with an exposure level of the color component of the corresponding pixel in the second image, and wherein the exposure level of the color component of the pixel in the first image represents a deviation of the value of the color component of the pixel from a predetermined value, and the exposure level of the color component of the corresponding pixel in the second image represents a deviation of the value of the color component of the corresponding pixel from the predetermined value, wherein the predetermined value is a middle value of a value range of the first image and the second image.