1. A computer-implemented diagnostic method for color stability in an image printing system, wherein the method is implemented in a computer system comprising one or more processors configured to execute one or more computer program modules, the method comprising: printing a test pattern on output media by forming an image on an image transfer surface and transferring the image on the image transfer surface to the output media, the test pattern comprising a plurality of prints and having a predetermined area coverage; measuring, during the printing of the test pattern, the image of the test pattern on the image transfer surface using one or more image transfer surface sensors to obtain one or more image transfer surface signals, wherein the image transfer surface signals are representative of a characteristic of the image of the test pattern on the image transfer surface; measuring a printed image of the test pattern using a printed image sensor to obtain a printed image signal, wherein the printed image signal is representative of a characteristic of the printed image of the test pattern; calculating correlation functions for the one or more image transfer surface signals and the printed image signal; and analyzing the correlation functions for the one or more image transfer surface signals and the printed image signal to identify a subsystem in the image printing system that causes variations in color within the plurality of prints of the test pattern.