This can be seen in the offline version on fixtures like the VL3500. Set all CMY params to zero, use Color Format repeatedly (a looped macro set to 1 second works well here), and eventually all CMY values will equal 100 (black). Obviously the conversion formulas (HS->CMY , CMY ->HS) are not reciprocal. Also there seems to be an implied Brightness parameter that is now hidden, which explains how all three colors get to 100, and why there is a "Bright. to Full" button on the ML display.
[edited by: 33boardop at 8:20 PM (GMT -6) on Mon, Apr 19 2010]