I just did a quick test drive of mupad, and was fairly impressed with the performance.  However, I tried to open a couple of CMYK PDFs and found that the color representation was exceedingly poor (in my photos it looks like everyone is infected with some sort of flesh eating bacteria).

Before you stop me, I recognize that CMYK is device independent and that CMYK to RGB conversion depends on the profile, my point is that the profile that is chosen by default may not be the best.

I took a look at the source, and I am honestly not sure what is going on there.  It looks like cmyktoxyz() converts cmyk to CIE XYZ using some undocumented color profile, but then those XYZ values are used as RGB values?  It's not clear to me.

At any rate, it seems to be that the default should use the SWOP profile (http://www.swop.org/specification), which appears to be the default Adobe CMYK profile.  This provides a precise transform to CIELAB, which has a precise transform to RGB.

Of course, just using the SWOP profile won't work perfectly either, since other people would use different profiles.  But that seems like it would be a much better default profile then whatever is being used now.