1. A method implemented by one or more devices, the method comprising: receiving, by the one or more devices, first and second images; determining, by the one or more devices, a first histogram based on hue values associated with pixels in the first image; determining, by the one or more devices, a second histogram based on hue values associated with pixels in the second image; determining, by the one or more devices, a color difference metric based on a comparison between the first and second histograms, the color difference metric relating to an objective measure of color differences between the first and second images; and outputting or storing, by the one or more devices, the color difference metric.