1. A computer-implemented method for determining at least one image quality metric of a color marking device, the method comprising: measuring image noise from target color patches with an in-line sensor, where said target color patches are on-belt single-separation device-dependent colors for a target marking device; calculating a given on-belt image quality metric from said image noise which has been measured; estimating an image quality metric on-paper for said target color marking device with said on-belt image quality metric using a correlation model for single-separation device-dependent colors for on-belt measurement with on-paper measurement; determining the device-independent color variations by using said estimated image quality metrics for said single-separation device-dependent colors; estimating a device-dependent color variation as a function of area-coverage for each colorant using said device-independent color variation; estimating at least one device-independent color variation for at least one device-dependent multi-separation color of interest, based upon said device-dependent color variation as a function of area-coverage; and determining said image quality metric of said target marking device by using said estimated device-independent color variation for said at least one device-dependent multi-separation color of interest.