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On high-res scans (600 dpi or more), I find it quite useful to eliminate coarse color noise with a denoising soft based on noise profiling such as noise ninja before finishing-up with greyc. These are quite effective against images with large uniform areas.
These softwares sometimes cause minor color shifts due to their nature though, so be careful.

IMO, for a quick comparison, noise ninja is based purely on noise profiling and employs little or no structure detection, while Neatimage employs both. And greyc doesn't have profiling and relies solely on blurring along structure vectors. I got to these conclusions by inspecting their resulting artifacts, and have no proof so don’t take it too seriously.

Note that it doesn’t imply that I think Neatimage does better job. In fact I use noiseninja + greyc myself.

Also something off topic: In fact all these methods eliminate noise by simply averaging, which is the only method against random ISO noise produced by CCD sensors. In fact CMYK screening is not random, it should be possible to solve back to a much better source image if we take the periodicity of screening into account. But it seems no easy job, maybe enough for a master degree research project… Would a nice lab please have someone work on this?....