No—- not blessed to be a collaborator (unless it’s someone else copying my putzing). I just translated the RGB - CMYK algorithm from this CodeProject page into a nodular equation. I found that via the WikiPedia article on the CMYK Color Model(which I’d bookmarked).
It’s (ostensibly) the procedural equivalent of
Black = minimum(1-Red,1-Green,1-Blue)
Cyan = (1-Red-Black)/(1-Black)
Magenta = (1-Green-Black)/(1-Black)
Yellow = (1-Blue-Black)/(1-Black)
and I figured the node tree would be useful to others as well.
BTW: Has anyone else found ShaderMixer to be more finicky when custom nodes are left grouped? I always seem to get better results when I ungroup them (or I did with D|S-P 4.0, and carried the habit over to 4.5).