If the inputs are PBM or PGM, pnmpsnr prints the PSNR of the luminance only. Otherwise, it prints the separate PSNRs of the luminance, and chrominance (Cb and Cr) components of the colors.
The PSNR of a given component is just the mean square difference of the component, comparing each pixel with the pixel in the same position of the other image, expressed as a decibel value. For the purposes of this computation, components are normalized to the scale [0..1].