I might be an outlier, but, I spent a few hours last weekend looking for a color picker that had CMYK support.

Although, maybe I'll send a pull request your way, if I find the time to add it in.. should just be a matter of colorUsingColorSpace: and {cyan, magenta, yellow, black}Components of NSColor and a quick addition to your NSColorFormatter.

(// edit: other than that, this looks pretty awesome.)