1. A method, comprising: for each of multiple given pixels of a color image having multiple color channels, determining a respective local difference measure value from a value of the given pixel and a respective value of at least one other pixel in a local neighborhood of the given pixel for each of multiple of the color channels; for each of the given pixels, ascertaining a respective color correlation value measuring deviation of the respective local difference measure values determined for the multiple color channels from a reference; and calculating a level of noise in the color image from one or more of the ascertained color correlation values; wherein the determining, the ascertaining, and the calculating are performed by a machine.