1. A method of assessing the operation of a color printing system, comprising: printing a multi-sheet test job with corresponding test points on each sheet distributed vertically and horizontally over the sheet, the test points comprising test markings for enabling measurement of horizontal and vertical color plane registration for each of multiple printing colors; measuring the printed test markings at the test points to determine for said printing colors a multiplicity of horizontal and vertical color plane misregistration values; analyzing the multiplicity of misregistration values to derive a plurality of misregistration parameters providing different respective views of misregistration behavior across the test sheets; and providing an output based on the misregistration parameters, wherein each test sheet includes a printed vertical series of markings for enabling measurement of a sequence of vertical misregistration values that are ‘theta values’ for at least some of the printing colors, the measuring step comprising measuring the vertical series of markings to determine a sequence of theta values for one or more of the printing colors, and the analyzing comprising deriving from the theta values theta-related parameters including the amplitudes of spatial frequency components present in the theta values.