1. A method of determining a time to replace a developing solution in a wet electrophotographic printer including developing solutions of predetermined colors, the method comprising: providing a reference card including sample patches of standard colors for the predetermined colors of the developing solutions, sample patches of test colors into which the standard colors have been changed due to contamination with other colors, and standard color differences &Dgr;E* calculated from standard color values E s * corresponding to the respective standard colors and measured standard contamination values E c * corresponding to the respective test colors; inputting information about test patches for the predetermined colors into the printer and printing the test patches; and comparing the sample patches with the test patches to determine whether to replace one of the developing solutions.